A somewhat random selection of my all time favourites:
Anything by Franz Kafka - esp. The Trial, The Castle, Metamorphosis.
Fjodor Dostojefski - Crime and Punishment, Notes from Underground, The Brothers Karamazov.
Lewis Carroll - Alice in Wonderland, Through the Looking Glass
Edgar Allen Poe - Tales of Mystery and Imagination
Aldous Huxley - Eyeless in Gaza
Bram Stoker - Dracula
Mary Shelley - Frankenstein or The Modern Prometheus
Tove Jansson - all the Moomintroll books (yes I still read them)
Douglas Hofstadter - Gödel, Escher, Bach: an Eternal Golden Braid, Metamagical Themas, I am a Strange Loop
David Hume - An Enquiry Concerning Human Understanding
Adam Smith - The Theory of Moral Sentiments
Richard Dawkins - The Selfish Gene
Jonathan Swift - Gulliver's Travels
Anything by Bertrand Russell (except perhaps Principia Mathematica)
Sir James Frazer - The Golden Bough
Anything by Daniel Dennett
Don Cupitt - The Sea of Faith
Erwin Schrödinger - What is Life?
Eric Hobsbawm - The Age of Revolution, The Age of Capital, The Age of Empire, The Age of Extremes.
Umberto Eco - Foucault's Pendulum
Marshall McLuhan - Understanding Media, The Medium is the Massage
Jean Genet - Diary of a Thief
Susan Blackmore - The Meme Machine
William Morris - News from Nowhere