Reading Impact

Which book that you’ve read has had the most profound effect on you? And why?

From Frank
I groaned when I read this question. Not because it isn't a good question - it's a great one, in fact. No, my reaction was simply because it is an impossible question to answer. 
How can I choose just one?
It's time like these that I almost wish I was a religious fundamentalist. I could point to the guiding religious tome of my faith, snap my fingers, and be pressing PUBLISH on this post at 75 words in. But I do...