After embellishing the 19th century with alternative histories and fantastic developments in four previous novels, beginning with her best-selling debut, “The Watchmaker of Filigree Street,” Natasha Pulley grounds her latest work in an actual 20th-century event. “The Half Life of Valery K” takes off from a 1957 nuclear explosion in the Soviet Union, which blasted mortally dangerous levels of radiation into the atmosphere, and the ensuing coverup by the Soviet government. Exhibiting all the storytelling skills that made her earlier books readable and popular, Pulley offers a piercing study of how a police state deforms individual psychologies, personal relationships and professional ethics.