Do you worry that you've lost patience for anything longer than a tweet? If so, you're not alone. Digital-age pundits warn that as our appetite for books dwindles, so too do the virtues in which printed, bound objects once trained us: the willpower to focus on a sustained argument, the curiosity to look beyond the day's news, the willingness to be alone. The shelves of the world's great libraries, though, tell a more complicated story.