Eccentric old customers, books in code, secret societies, and clandestine trips to Google’s headquarters—these are things that Clay never expected before he stepped foot into Mr. Penumbra’s bookstore. When he loses his job as a web designer for a bagel company and becomes a bookseller in a dusty old building in San Francisco, he is tasked with keeping a log of the customers’ unpredictable visits: “You must keep precise records of all purchases. Time. Amount. The customer’s appearance. His state of mind. How he asks for the book. How he receives it. Does he appear to be injured. Is he wearing a sprig of rosemary on his hat. And so on.” A curious peek into one of these books leads Clay on an exciting cross-country quest to solve Mr. Penumbra’s puzzle.