Well, if we're talking about books that have the potential to make a big impact, or change your outlook on things, or shake you up, or just make you really, really think, then these books did that for me:
Zen Mind, Beginner's Mind by Shunryu Suzuki
Being Zen by Ezra Bayda
The Dharma Bums by Jack Kerouac
The Way of the Peaceful Warrior by Dan Millman
Skinny Legs and All by Tom Robbins
The End of Faith: Religion, Terror and the Future of Reason by Sam Harris