I last read Joanne Harris when she hit the big time with Chocolat over two decades ago. I guess I typecast her thereafter and ignored a steady stream of novels in diverse styles and genres, including a series of standalone thrillers set in a fictional boys’ school, St Oswald’s. The third in this loose trilogy, “A Narrow Door,” captivates from the very first chapters, spilling a deliciously twisty yarn about the new female headmaster (one of the two voices present) regaling her puzzling familial and career pasts from decades ago to an ancient St Oswald’s teacher (the other alternative voice). The stimulus is a body under the sports ground, a body from the past. Harris is a salty, evocative storyteller, clearly enjoying laying down clues for the crime fiction reader, and her two narrative voices are both strong and true. I like to think I can figure out a mystery’s potential paths by the book’s midpoint, but A Narrow Door baffled me as much as it gripped, and I read in a flurry. The novel climaxes with a thoroughly satisfying and devious confluence of past lives and the present.