The God Delusion One of the best selling books on atheism of all time, and for good reason. The arguments are well formed and well researched and it makes for a thought provoki
The End of Faith: Religion, Terror, and the Future of Reason
Religulous A documentary about Bill Maher's atheism, his struggles with his family and his unique take on religion.
On the Origin of Species: By Means of Natural Selection (Dover Thrift Editions) The most important book ever written.
Surely You're Joking, Mr. Feynman! (Adventures of a Curious Character) This book doesn't have anything to do with atheism per se, but should be read by everyone. Absolutely fascinating, intriguing and amusing in equal measure.