AP Photo/Kathy Willens Author Joan Didion at her New York apartment in 2005. T he images are of water, the murky kind, the kind with snakes and water moccasins and alligators and mud, the kind in which the distinction between swamp and stagnant river is difficult to discern. Water is a familiar Joan Didion obsession, but as she journeys through the Gulf South in the summer of 1970, she begins to sense that, cut off from the cultural centers of the East and West coasts, steeped in a section of America suffocating in its past, she is “underwater in some real sense.” In Didion’s account of this road trip, the light, which on the highway to Biloxi, Mississippi, is “entirely absorbed by what it strikes,” is also an element of this haunting nightmare-scape, one with crushed oyster shells crunching underfoot at a gas station, and whose residents have a “vertiginous preoccupation with race, class, heritage, style, and the absence of style.” As she and her husband set off from New Orleans,...