There is a House with 7,678 halls; in the House lives the eponymous protagonist of Susanna Clarke’s new novel, Piranesi. The amnesiac narrator knows nothing beyond this architecture, which inhabits and obsesses him—this mansion with great staircases, courtyards, and marble statuary, where, depending on the level reached, you could be in the domain of the clouds (Upper Halls), conversing with birds (Middle Halls), or plunged into an underwater ecosystem (Lower Halls). Piranesi cohabits this space with human remains, 13 skeletons whom he domesticates with names and attributes: the Biscuit-...