Slaughter House Five by Kurt Vonnegut
The Grapes of Wrath by John Steinbeck
The Heart is a Lonely Hunter by Carson McCullen
Catch-22 by Joseph Heller
Brave New World by Aldous Huxley
1984 by George Orwell
East of Eden by John Steinbeck
The Lord of the Rings trilogy by J. R. R. Tolkien
Life After God by Douglas Coupland
Ain't Nobody's Business if You Do: The Absurdity of Consensual Crimes in Our Free Country by Peter McWilliams
Beginnings: The Story of Origins, Mankind, Life, the Earth, the Universe by Isaac Asimov
Demonic Males: Apes and the Origins of Human Violence by Richard Wrangham and Dale Peterson [This book is great, but I'm also interested in how social evolution has shaped human behavior, which, unfortunately, this book fails to address.]
The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory by Brian Greene