I was going to put down "The Selfish Gene" myself. I remember picking it up in high school and hating the first few pages. I read the first chapter in college, and I didn't get what all the fuss was about. After college, I picked up the book, and was blown away! In retrospect, my younger self wasn't ready for it
For younger readers, or those who prefer a lighter reader, Dawkins "The Greatest Show On Earth" covers similar material in a more accessible (if less rigorous) manner. I forgot to mention it in my own recommendations, but it is one of the books I recommend at every opportunity.
