DISCUSSION: What Books Have Changed Your Way of Thinking?

The Atlantic asked its writers and editors to name, in the words of their headline, A Book That Changed How I Think.  Or, as Gal Beckerman put it, A Book That Was Like Putting on ‘a New Set of Glasses.

So, I put it to you: name a book that was not only informative or enjoyable but paradigm-shifting, that changed your outlook or perspective on. . .either life, or some subset of it.  (The Bible will definitely do that, but what else?)