Some of my favorites are:
Zen and the Art of Motorcycle Maintenance by Robert Pirsig
Siddhartha by Herman Hesse
Dune by Frank Herbert
The Plague by Albert Camus
The Last Temptation of Christ by Nikos Kazantzakis
Anything by Kurt Vonnegut
Anything by Fyodor Dostoevsky
Zen and the Art of Motorcycle Maintenance by Robert Pirsig
Siddhartha by Herman Hesse
Dune by Frank Herbert
The Plague by Albert Camus
The Last Temptation of Christ by Nikos Kazantzakis
Anything by Kurt Vonnegut
Anything by Fyodor Dostoevsky