Disregarding what's already in this thread (Vonnegut!):
Ken Kesey: One Flew Over The Cuckoo's Nest. The Grandfather of LSD culture, but also an incredible author. The visceral descriptions and unreliable narrator make for a very interesting read to say the least. Then there's the colorful cast of characters...
Joseph Heller Catch-22. A dark comedy with a Kafkaesque bent. Reflects upon a lot of society in the modern age.
Albert Camus: The Stranger. The definitive piece of absurdist fiction. Also very short and exciting.
Jean-paul Sartre: No Exit. OK, yes I like French existentialists. No Exit (or In Camera, The Others, or a couple other translations iirc) is the source of the oft-misunderstood quote "HELL IS OTHER PEOPLE". A short play with characters who find themselves spending an afterlife together.
Thomas Pynchon: V and Gravity's Rainbow. Haven't read the rest of his work, but these are brilliant, wide ranging, complex woven narratives featuring masses of interconnected characters and events over years. Taking on one of these novels is a journey itself with their dense prose (and occasional lyrical interludes), confusing subplots and gargantuan length. But it's a journey full of rewards as well.
and on a slightly different note
Douglass Hofstadter: Gödel, Escher, Bach. A description of elements of number theory and Gödel's Incompleteness Theorum and application to computing, art, consciousness, music, puzzles, and more. Also poetic interludes.