The opening scene is perfection. We meet the eponymous heroine of Diane Johnson’s latest novel, “Lorna Mott Comes Home,” as she rides in the back of a taxi, en route to the train station in Lyon. Lorna, an American woman “of a certain age,” asks the driver to stop so she can observe the aftermath of a mudslide that has unearthed coffins, bursting them open and exposing corpses, bones and “a huge, sticky hillock of treacherous clay” in the village of Pont-les-Puits, where she has lived with her French husband for 20 years.