You'll notice that lots of other people are suggesting other Dawkins books. He has written lots of books on various facets of evolution, with most exploring a new, interesting portion of the subject. (I would recommend reading them in roughly chronological order; The Blind Watchmaker is a good follow-up to The Selfish Gene.) If you're going to stay at the popsci level, that's where I'd stay; if you're willing to read textbooks, that's probably a bit better use of your time.