Darwin's Dangerous Idea by Daniel Dennett, when I was 15, 25 years ago. I was already less than sympathetic to the whole God idea, but that book was the revelation that made it all click. I still remember the moment, p.53 or so, when he explained how a simple algo and a few billion years could explain all biology we see around us. As a kid, i was simply stunned,i immediately embraced the idea and it's consequences and never looked back.