Agatha Christie wrote her first detective story, The Mysterious Affair at Styles, in 1920. Thereafter, and for over half a century, she was the most popular purveyor of the genre. During that time she wrote works that would not fit quite as well within the narrowest definition of the genre. But detective fiction is a form that loses definition in proportion as it extends beyond its intentional narrowness—a truism confirmed by the lasting appeal of even as rudimentary a work as The Mysterious Affair a...