Imagine that you have ditched your laptop and turned off your smartphone. You are beyond the reach of YouTube, Facebook, e-mail, text messages. You are in a Twitter-free zone, sitting in a taxicab with a copy of “Rapt,” a guide by Winifred Gallagher to the science of paying attention. The book’s theme, which Ms. Gallagher chose after she learned she had an especially nasty form of cancer, is borro