The (book's) backstory:
Father Gary Thomas was working as a parish priest in California when he was asked by his bishop to travel to Rome for training in the rite of exorcism. Though initially surprised, and slightly reluctant, he accepted this call, and enrolled in a new exorcism course at a Vatican-affiliated university, which taught him, among other things, how to distinguish between a genuine possession and mental illness. Eventually he would go on to participate in more than eighty exorcisms as an apprentice to a veteran Italian exorcist. His experiences profoundly changed the way he viewed the spiritual world, and as he moved from rational skeptic to practicing exorcist he came to understand the battle between good and evil in a whole new light. Journalist Matt Baglio had full access to Father Gary over the course of his training, and much of what he learned defies explanation.
Here's a 23-page sample to give you a sense of what we're dealing with here. Demons, expulsion rites, the darkest side of Catholicism, and the source material for what promises to be a chilling film.