Here are of the most challenging and intellectually demanding books in literature:

"Finnegans Wake" by James Joyce (1939):

Often considered one of the most difficult novels ever written, "Finnegans Wake" is a linguistic labyrinth. Joyce employed complex wordplay, multiple languages, and a dreamlike narrative structure, making it a formidable read.
"Ulysses" by James Joyce (1922):

Another masterpiece by Joyce, "Ulysses" is known for its intricate stream-of-consciousness narrative and allusions to various literary works. It follows the experiences of Leopold Bloom over a single day in Dublin.
"In Search of Lost Time" by Marcel Proust (1913-1927):

A monumental seven-volume work, Proust's magnum opus explores themes of memory, time, and art. Its dense prose and introspective narrative demand careful reading and reflection.
"Being and Time" by Martin Heidegger (1927):

A cornerstone of existentialist philosophy, Heidegger's work delves into the nature of existence and time. It's known for its complex terminology and profound philosophical inquiries.
"Critique of Pure Reason" by Immanuel Kant (1781):

Kant's philosophical treatise redefined metaphysics and epistemology. It addresses the nature of human knowledge and experience, but its dense prose and intricate arguments can be challenging.
"The Phenomenology of Spirit" by Georg Wilhelm Friedrich Hegel (1807):

A foundational work in German idealism, Hegel's book explores the evolution of human consciousness and spirit. Its intricate dialectical method and philosophical depth require careful study.
"Gravity's Rainbow" by Thomas Pynchon (1973):

Pynchon's postmodern novel is known for its complex plot, numerous characters, and intricate symbolism. It weaves together themes of war, technology, and paranoia.
"The Sound and the Fury" by William Faulkner (1929):

Faulkner's modernist novel is told from multiple perspectives and employs a nonlinear narrative. It explores the decline of a Southern aristocratic family, demanding close attention from readers.
"One Hundred Years of Solitude" by Gabriel García Márquez (1967):

This magical realist novel weaves a complex tapestry of characters and events across generations in the fictional town of Macondo. Its blending of reality and fantasy can be challenging to interpret.
"The Brothers Karamazov" by Fyodor Dostoevsky (1880):

Dostoevsky's philosophical novel explores themes of faith, morality, and free will through the lives of the Karamazov brothers. It's dense with philosophical discussions and complex characters.
"The Divine Comedy" by Dante Alighieri (c. 1320):

Dante's epic poem takes readers on a journey through Hell, Purgatory, and Heaven. Its allegorical and theological content, as well as its medieval Italian language, can be daunting.
"Absalom, Absalom!" by William Faulkner (1936):

Faulkner's novel is renowned for its complex narrative structure and exploration of the Southern Gothic tradition. It examines the legacy of Thomas Sutpen and his family in the American South.
"The Recognitions" by William Gaddis (1955):

A complex and erudite novel, Gaddis's work explores themes of art, forgery, and authenticity. It features dense prose and references to various cultural and historical elements.
"Moby-Dick" by Herman Melville (1851):

Melville's novel is a multifaceted exploration of obsession, morality, and the human condition. It includes detailed descriptions of whaling and maritime life that can be challenging for readers.
"The Trial" by Franz Kafka (1925):

Kafka's novel is a surreal exploration of bureaucracy and existential anxiety. It follows the nightmarish experiences of Josef K., who is arrested and put on trial without knowing the charges against him.
"War and Peace" by Leo Tolstoy (1869):

Tolstoy's epic novel spans the Napoleonic Wars and explores the lives of numerous characters. Its length and historical depth make it a formidable literary undertaking.
"The Myth of Sisyphus" by Albert Camus (1942):

Camus's philosophical essay delves into the concept of the absurd and the search for meaning in a seemingly indifferent universe. It challenges readers to confront existential questions.
"The Book of Disquiet" by Fernando Pessoa (posthumously, 1982):

Pessoa's fragmentary and introspective work is presented as the diary of a semi-heteronym named Bernardo Soares. It reflects on existence, dreams, and the nature of self.
"The Structure of Scientific Revolutions" by Thomas Kuhn (1962):

Kuhn's influential work in the philosophy of science challenges conventional views on scientific progress. It introduces the concept of paradigm shifts and can be intellectually demanding.
"Gödel, Escher, Bach: An Eternal Golden Braid" by Douglas Hofstadter (1979):

A non-fiction work that explores the connections between mathematics, art, and music, Hofstadter's book is known for its intricate interplay of ideas and concepts.
These books represent a range of literary and philosophical challenges, requiring readers to engage deeply with complex ideas, innovative narrative structures, and profound themes. While demanding, they offer rich rewards to those who persevere in their reading.

"The Trial and Death of Socrates" by Plato (c. 399 BC):

This philosophical work presents the trial and execution of the ancient Greek philosopher Socrates. It delves into Socrates' defense of his beliefs and his acceptance of his death sentence.
"The Critique of Practical Reason" by Immanuel Kant (1788):

Kant's philosophical treatise explores moral philosophy and the concept of practical reason. It deals with complex ethical ideas and Kant's categorical imperative.
"The Magic Mountain" by Thomas Mann (1924):

Mann's novel is a dense exploration of time, illness, and the intellectual climate in Europe before World War I. It's known for its intricate discussions and symbolism.
"The Golden Bough" by James George Frazer (1890):

A monumental work in anthropology and comparative religion, Frazer's book explores the mythologies and rituals of cultures worldwide. Its encyclopedic scope and academic language can be challenging.
"The Interpretation of Dreams" by Sigmund Freud (1899):

Freud's groundbreaking work on psychoanalysis delves into the interpretation of dreams and the unconscious mind. It introduces complex psychological concepts and theories.
"The Cantos" by Ezra Pound (1917-1969):

Pound's long poem is a modernist masterpiece that weaves together history, literature, and personal reflections. Its fragmented and allusive style demands careful reading.
"Tractatus Logico-Philosophicus" by Ludwig Wittgenstein (1921):

Wittgenstein's philosophical tract explores language, logic, and the limits of meaningful expression. It's known for its concise and often enigmatic statements.
"The Phenomenology of Perception" by Maurice Merleau-Ponty (1945):

Merleau-Ponty's philosophical work delves into phenomenology and the relationship between perception and consciousness. It's a dense exploration of human experience.
"The Gay Science" by Friedrich Nietzsche (1882):

Nietzsche's philosophical work introduces the concept of the "eternal recurrence" and explores themes of nihilism and the will to power. It's characterized by its aphoristic style.
"Being and Nothingness" by Jean-Paul Sartre (1943):

Sartre's existentialist treatise delves into the nature of human existence, freedom, and responsibility. It presents complex philosophical ideas and existentialist themes.
"The Phenomenology of Mind" by Georg Wilhelm Friedrich Hegel (1807):

A foundational work in the history of philosophy, Hegel's book explores the development of human consciousness and self-awareness. Its dialectical method and terminology can be challenging.
"The Plague" by Albert Camus (1947):

Camus's allegorical novel explores the human condition in the face of an epidemic. It delves into existentialist themes and moral dilemmas.
"The Theory of Communicative Action" by Jürgen Habermas (1981):

Habermas's extensive work in critical theory and sociology presents complex ideas related to communicative rationality and the public sphere.
"The Life and Opinions of Tristram Shandy, Gentleman" by Laurence Sterne (1759-1767):

Sterne's novel is a postmodernist precursor known for its playful narrative style, digressions, and metafictional elements. It challenges traditional narrative conventions.
"The Principles of Mathematics" by Bertrand Russell (1903):

Russell's philosophical work delves into the foundations of mathematics and logic. It presents complex discussions on the philosophy of mathematics.
"The Archetypes and the Collective Unconscious" by Carl Jung (1959):

Jung's exploration of archetypes and the collective unconscious introduces complex psychological concepts and symbolism.
"The Birth of Tragedy" by Friedrich Nietzsche (1872):

Nietzsche's early philosophical work explores the duality of the Apollonian and Dionysian in Greek tragedy and its relevance to art and culture.
"The Making of Americans" by Gertrude Stein (1925):

Stein's experimental novel is known for its repetitive prose and explorations of identity, family, and American culture.
"The Philosophy of Right" by Georg Wilhelm Friedrich Hegel (1821):

Hegel's work in political philosophy presents a complex analysis of rights, ethics, and the state, making it challenging for readers.
"Illuminations" by Walter Benjamin (posthumously, 1968):

A collection of essays and reflections by the German philosopher and critic, Benjamin's work touches on diverse topics and can be dense in its theoretical discussions.
These books span a range of disciplines, including philosophy, literature, psychology, and sociology, and they challenge readers with their intricate ideas, philosophical depth, and complex narratives.

"The Phenomenology of Spirit" by Georg Wilhelm Friedrich Hegel (1807):

Hegel's influential work is known for its complex exploration of human self-consciousness, dialectical reasoning, and historical development of ideas. It's a cornerstone of German idealism.
"The Structure of Behavior" by Maurice Merleau-Ponty (1942):

Merleau-Ponty's philosophical work delves into the relationship between perception and action, challenging traditional views of mind and body.
"The Logic of Scientific Discovery" by Karl Popper (1934):

Popper's seminal work in the philosophy of science introduces the concept of falsifiability and addresses the nature of scientific progress and knowledge.
"The Use of Bodies" by Giorgio Agamben (2014):

Part of Agamben's ongoing exploration of biopolitics and sovereignty, this philosophical work challenges contemporary views on the body and its political implications.
"Being and Time" by John Macquarrie and Edward Robinson (1962) [English Translation]:

Martin Heidegger's original "Being and Time" is already on the list, but this is a notable English translation known for its depth and difficulty in capturing Heidegger's original ideas.
"The Concept of Mind" by Gilbert Ryle (1949):

Ryle's philosophy of mind challenges the traditional Cartesian dualism and introduces the concept of "category mistakes" in understanding mental processes.
"The Doctrine of Being" by Georg Wilhelm Friedrich Hegel (1831):

Another significant work by Hegel, it explores themes of metaphysics, ontology, and the nature of reality. It's known for its systematic and intricate philosophical arguments.
"The Phenomenology of Internal Time-Consciousness" by Edmund Husserl (1928):

A foundational work in phenomenology, Husserl's book delves into the nature of consciousness and the experience of time, presenting complex philosophical ideas.
"The Order of Things" by Michel Foucault (1966):

Foucault's exploration of epistemes and the history of knowledge challenges traditional ideas about the structure of knowledge and classification.
"The Essence of Christianity" by Ludwig Feuerbach (1841):

Feuerbach's philosophical work challenges religious beliefs and explores the idea that God is a projection of human qualities.
"The Open Society and Its Enemies" by Karl Popper (1945):

Popper's political philosophy challenges authoritarian ideologies and explores the concept of the open society, demanding careful consideration of political ideas.
"The World as Will and Representation" by Arthur Schopenhauer (1818):

Schopenhauer's metaphysical work presents his philosophy of will and representation, offering complex ideas about reality, perception, and human desires.
"Ideas I: Investigation on the Sources of Knowledge" by Edmund Husserl (1913):

Another important work by Husserl, it examines the foundations of knowledge, epistemology, and the nature of consciousness.
"The Will to Power" by Friedrich Nietzsche (posthumously, 1901):

A collection of Nietzsche's unpublished notes and aphorisms, this book explores themes of power, culture, and morality.
"The Philosophy of Right" by Karl Marx (1843-1844):

Marx's philosophical work addresses political and social theory, presenting complex ideas about justice, the state, and human liberation.
"The Logic of Sense" by Gilles Deleuze (1969):

Deleuze's philosophical work challenges conventional notions of language and sense, offering a complex exploration of signification and events.
"Being and Time" by Jean-Paul Sartre (1966) [English Translation]:

Sartre's original "Being and Nothingness" is already on the list, but this is a notable English translation that captures Sartre's existentialist ideas.
"The Birth of the Clinic" by Michel Foucault (1963):

Foucault's exploration of the history of medicine and the clinic challenges traditional ideas about knowledge, power, and institutional structures.
"Being and Event" by Alain Badiou (1988):

Badiou's philosophical work introduces the concept of "event" and explores themes of ontology, ethics, and mathematics.
"The Philosophy of Symbolic Forms" by Ernst Cassirer (1923-1929):

Cassirer's multi-volume work presents a philosophical exploration of symbolic forms, including language, myth, and science, demanding a deep understanding of human culture and cognition.
These challenging books span various philosophical and intellectual domains, pushing the boundaries of thought and demanding rigorous engagement from readers interested in exploring complex ideas and concepts.

"The Life and Opinions of the Tomcat Murr" by E.T.A. Hoffmann (1820-1822):

Hoffmann's novel is a complex work of metafiction, blending the life stories of two characters, a human musician and a tomcat. It challenges traditional narrative structures.
"The Ethics of Ambiguity" by Simone de Beauvoir (1947):

De Beauvoir's philosophical work delves into existentialism and ethics, exploring the concept of freedom and ambiguity in human existence.
"Theodor Adorno: Negative Dialectics" by Theodor W. Adorno (1966):

Adorno's philosophical treatise challenges traditional dialectical thinking and presents complex ideas about language, history, and contradiction.
"The Concept of the Political" by Carl Schmitt (1932):

Schmitt's political philosophy explores the nature of political concepts and sovereignty, offering complex ideas about state and politics.
"The Logic of Hegel" by Georg Wilhelm Friedrich Hegel (1812-1813):

This work by Hegel explores his dialectical method and complex ideas about logic, metaphysics, and philosophy of mind.
"The Raw and the Cooked" by Claude Lévi-Strauss (1964):

Lévi-Strauss's anthropological work introduces structuralism and explores the structure of myths and cultural phenomena in complex ways.
"The Concept of Anxiety" by Søren Kierkegaard (1844):

Kierkegaard's philosophical exploration of anxiety delves into existential themes, faith, and the human condition.
"The Making of the Representative for Planet 8" by Doris Lessing (1982):

Lessing's science fiction work is known for its complex narrative structure and themes of environmentalism, colonization, and consciousness.
"The Concept of Mind" by Ryle, Gilbert (1949):

Gilbert Ryle's philosophy of mind challenges traditional dualism and introduces the concept of "category mistakes" in understanding mental processes.
"Nausea" by Jean-Paul Sartre (1938):

Sartre's novel explores themes of existentialism, absurdity, and the human experience, often through the lens of the protagonist's existential crisis.
"Critique of Dialectical Reason" by Jean-Paul Sartre (1960):

Sartre's philosophical work addresses historical materialism and the complexities of social and political theory, demanding a deep engagement with dialectical reasoning.
"The Logic of Scientific Discovery" by Karl Popper (1934):

Karl Popper's philosophy of science challenges traditional views of scientific progress and introduces the concept of falsifiability.
"The History of Sexuality" by Michel Foucault (1976-1984):

Foucault's multi-volume work explores the history of sexuality, challenging conventional ideas about power, knowledge, and the body.
"The Transhumanist Reader" edited by Max More and Natasha Vita-More (2013):

A collection of essays on transhumanism, this book presents complex ideas about human enhancement, technology, and the future of humanity.
"The Critique of Pure Reason" by Immanuel Kant (1781):

Kant's philosophical treatise redefined metaphysics and epistemology, addressing the nature of human knowledge and experience.
"The Primacy of Perception" by Maurice Merleau-Ponty (1964):

Merleau-Ponty's philosophical work explores the relationship between perception and embodiment, presenting complex ideas about phenomenology and consciousness.
"The Logic of Sense" by Gilles Deleuze (1969):

Deleuze's philosophical work challenges conventional notions of language and sense, offering a complex exploration of signification and events.
"Being and Time" by Jean-Paul Sartre (1966) [English Translation]:

Sartre's original "Being and Nothingness" is already on the list, but this is a notable English translation that captures Sartre's existentialist ideas.
"The Birth of the Clinic" by Michel Foucault (1963):

Foucault's exploration of the history of medicine and the clinic challenges traditional ideas about knowledge, power, and institutional structures.
"The Will to Power" by Friedrich Nietzsche (posthumously, 1901):

A collection of Nietzsche's unpublished notes and aphorisms, this book explores themes of power, culture, and morality.
These books represent a wide range of disciplines and philosophical approaches, demanding careful consideration and intellectual engagement from readers interested in exploring intricate ideas and complex concepts.

"The Ego and Its Own" by Max Stirner (1844):

Stirner's philosophical work challenges conventional notions of morality and individualism, introducing the concept of "egoism" in a complex and controversial manner.
"The Life and Times of Michael K" by J.M. Coetzee (1983):

Coetzee's novel explores themes of survival, identity, and apartheid in South Africa through a narrative that challenges traditional storytelling conventions.
"The Concept of Nature" by Alfred North Whitehead (1920):

Whitehead's philosophical work delves into the philosophy of science, metaphysics, and the concept of nature, presenting complex ideas about process philosophy.
"The Principles of Quantum Mechanics" by Paul Dirac (1930):

Dirac's foundational work in physics presents complex mathematical and conceptual ideas about quantum mechanics, shaping the field of quantum physics.
"The Archaeology of Knowledge" by Michel Foucault (1969):

Foucault's philosophical work explores the nature of knowledge and discourse, challenging traditional views on the organization of knowledge.
"Being and Nothingness" by Jean-Paul Sartre (1943):

Sartre's existentialist treatise delves into the nature of human existence, freedom, and responsibility, presenting complex philosophical ideas.
"The Accursed Share" by Georges Bataille (1949):

Bataille's philosophical exploration of excess, sacrifice, and economic theory introduces complex ideas about human behavior and culture.
"The Culture Industry" by Theodor W. Adorno and Max Horkheimer (1947):

Adorno and Horkheimer's critical theory work addresses mass culture and its impact on society, presenting complex ideas about commodification and media.
"Being and Event" by Alain Badiou (1988):

Badiou's philosophical work introduces the concept of "event" and explores themes of ontology, ethics, and mathematics.
"The Critique of Pure Reason" by Immanuel Kant (1781):

Kant's philosophical treatise redefined metaphysics and epistemology, addressing the nature of human knowledge and experience.
"Being and Time" by John Macquarrie and Edward Robinson (1962) [English Translation]:

Martin Heidegger's original "Being and Time" is already on the list, but this is a notable English translation known for its depth and difficulty in capturing Heidegger's original ideas.
"The Concept of Anxiety" by Søren Kierkegaard (1844):

Kierkegaard's philosophical exploration of anxiety delves into existential themes, faith, and the human condition.
"The Logic of Scientific Discovery" by Karl Popper (1934):

Popper's seminal work in the philosophy of science challenges traditional views of scientific progress and knowledge.
"The Logic of Hegel" by Georg Wilhelm Friedrich Hegel (1812-1813):

This work by Hegel explores his dialectical method and complex ideas about logic, metaphysics, and philosophy of mind.
"The Birth of the Clinic" by Michel Foucault (1963):

Foucault's exploration of the history of medicine and the clinic challenges traditional ideas about knowledge, power, and institutional structures.
"The Will to Power" by Friedrich Nietzsche (posthumously, 1901):

A collection of Nietzsche's unpublished notes and aphorisms, this book explores themes of power, culture, and morality.
"The Primacy of Perception" by Maurice Merleau-Ponty (1964):

Merleau-Ponty's philosophical work explores the relationship between perception and embodiment, presenting complex ideas about phenomenology and consciousness.
"The Concept of Mind" by Ryle, Gilbert (1949):

Gilbert Ryle's philosophy of mind challenges traditional dualism and introduces the concept of "category mistakes" in understanding mental processes.
"The Concept of Nature" by Alfred North Whitehead (1920):

Whitehead's philosophical work delves into the philosophy of science, metaphysics, and the concept of nature, presenting complex ideas about process philosophy.
"The Principles of Quantum Mechanics" by Paul Dirac (1930):
- Dirac's foundational work in physics presents complex mathematical and conceptual ideas about quantum mechanics, shaping the field of quantum physics.

These books represent a broad range of intellectual disciplines and philosophical perspectives, demanding rigorous engagement and deep exploration of complex ideas.