Although British novelist Robertson (Instruments of Darkness, 2011, etc.) has no difficulty resuming the engaging tone of her period series, this sequel, after its punchy prologue, has a noticeably slacker pace. Mrs. Harriet Westerman, one half of the detective duo, is preoccupied with the mental health of her naval captain husband James, wounded after capturing a French ship carrying a spy during the war with the American Rebels. Now Harriet and her forensic scientist friend Gabriel Crowther are invited by the British authorities to help trace the espionage links to London, starting with the examination of a body found floating in the Thames. These investigations, and the dark fears of a slum-dwelling fortune-teller, are the driving forces for some two-thirds of the story, and they’re not enough to sustain excitement, even when packed with atmospheric background and well-researched historical detail. More corpses follow, as well as a predictable, busy denouement, none of which diminishes the sense of a weakly plotted tale and a small cast of characters too conveniently connected.Background authenticity is a substitute for foreground thrills in a solid but less-enthralling follow-up. Book three is under way.