Darwin's Dangerous Idea: Evolution and the Meanings of Life by Daniel C. Dennett
If I had to choose one book as the single greatest thing I have ever read, Darwin's Dangerous Idea would be it. I guess this seems a bit dramatic/hyperbolic, but I'm being sincere. Really- buy it now, read it several times.
Daniel Dennett's book, Darwin's Dangerous Idea is a brilliantly written book about not just evolution, but the social implications of naturalism, science and evolution. Dennett's a brilliant and evocative philosopher, a witty public speaker and an apologist for reason and rationality. This book is a must read... absolutely.