Probably no detective story in history has met with such instantaneous success as ["Curtain"]…. Poirot dead? It seems incredible. The little Belgian detective had been most active between 1900 and 1904; by 1920, when he appeared in Christie's first novel, "The Mysterious Affair at Styles," he was officially retired. And yet he went on to star in 40 of his author's 86 books—which is about as firm a grip on immortality as a literary man can get. For this reason, his death comes as an unexpected jolt. (p. 91)

"Curtain" is one of Christie's most ingenious stories, a tour de force in which the lady who had bent all the rules of the genre before bends them yet again. Like all her stories, it is scrupulously honest. In a detective story, as in an allegory, much that happens—the concrete details that provide an illusion of reality—actually points to something else, and...