The Outsider - Albert Camus
A quickie but a goodie, The Outsider (aka The Stranger) is a massively influential book for men of a certain semi-intellectual, atheistic bent. I would classify myself in that group, I suppose, but I'm probably too old to be much changed by this tale of alienation from society. Catcher in the Rye was my youthful rebellion novel, and I still think of hypocrites as "phonies".
2 points of trivia:
1) My second book this month to have inspired an indie-pop classic
2) Dubya's choice of reading material this summer. I wonder how he felt about the depiction of colonialists murdering arabs for no discernible reason?