"The Selfish Gene" by Dawkins. This was a result of asking my Twitter friends to decide between this and Art of War for me, and this won overwhelmingly. It's been a great read so far, and I'm really looking forward to learning more about how evolution and computer science can mix.
The God Delusion was the first Dawkins book I read -- a Christmas gift from my mother. When I told her what it was about, she got made and said "that's not in the spirit of Christmas."
