I woke up thinking about Ron Hansen's majestic, mournful book "The Assassination of Jesse James by the Coward Robert Ford." The book had gotten into my head the way some songs do, repeating its rhythms and tones over and over. On a whim, I mentioned it on Twitter, added the searchable hashtag #booksthatchangedmyworld, and sat still for a moment. About three seconds later, the flood began—dozens and dozens of other people started listing books that had changed their worlds. They kept listing f...