For Steven A. LaChance, the possession case that inspired The Exorcist has always felt . . . incomplete. In this book, he shows how the official account crumbles under scrutiny, revealing instead a story of familial horror and spiritual corruption that implicates the highest levels of the Catholic Church. Known as the 1949 St. Louis Exorcism, the harrowing story of a possessed child and his terrified family was immortalized in The Exorcist . While...