O'Neill is one of my favorite anthropologists of religion.
Unforgivable, however, is not quite a work of anthropology. Its fictional elements bring to mind works like In Cold Blood by Truman Capote.
The book is an account of Father David Roney. Roney was one of the many, many abuser priests that racked the Catholic world in the 70s and 80s.
O'Neill's portraits of Roney's abuse and manipulation are sickening and rage-inducing. Yet, O'Neill manages not to reduce the book to a sensationalizing of an individual bad actor. Throughout the book, he ties in Roney's story to the Catholic Church as an institution with incentives not to talk about sex, and not to call out abuser priests.
This book is not for the faint of heart. But I believe that it is an accessible introduction for academics and the public alike who are interested in learning about clerical abuse.