In the winter of 1952, a thick smog blanketed London for five days, killing 12,000 people. And it provided a perfect cover for a killer named John Reginald Christie to terrorize the city from the ground. Kate Winkler Dawson tells the story of a particularly disturbing week in British history, which she writes about in “Death in the Air: The True Story of a Serial Killer, the Great London Smog, and the Strangling of a City” (Hachette Books).