“How to Be Good” is British author Nick Hornby’s third novel and, like his best work, “High Fidelity,” it’s stuffed with the kind of relationship nuance that can make him such a zip to read. But “How to Be Good” is a joyless book, and it’s the protagonists who drag it down. Katie is just a rewrite of Hornby’s charmingly adrift men–she’s the earner, she has the affair, she moves out–only without the charm. David, meanwhile, is just a prop to set Katie’s identity crisis in motion, transforming in a blink from the Grinch to Gandhi. In the end, “How to Be Good” only nips at the toes of big issues. What does goodness require? Don’t have affairs–sure, that’s easy. But then what? Give away spare change? Take in a homeless kid? After 300 pages, Hornby has brought us no closer to an answer.