With so many things now demanding our attention -- emails, Web sites, BlackBerry alerts, incoming text messages, Twitter tweets, Facebook updates, blogs, stock updates, and old- fashioned meetings and phone calls -- many of us . . .

What was I saying? Right: Many of us fret about losing our train of thought. "Life," says Winifred Gallagher, "is the sum of what you focus on." In "Rapt," she concentrates on what science has to tell us about the mind's capacities for paying attention.

Some...