What is this mysterious thing called self-control? When we fight an urge, it feels like a strenuous effort, as if there were a homunculus in the head that physically impinged on a persistent antagonist. We speak of exerting will power, of forcing ourselves to go to work, of restraining ourselves and of controlling our temper, as if it were an unruly dog. In recent years the psychologist Roy F. Baumeister has shown that the force metaphor has a kernel of neurobiological reality. In “Willpower,” he has teamed up with the irreverent New York Times science columnist John Tierney to explain this ingenious research and show how it can enhance our lives.