Rickman's main characters are Merrily Watkins (the diocesan exorcist), her 18 year old daughter, Jane, and their friends. In this book, Watkins Snr.'s approached over an issue by a colleague who's returned to the SAS as pastor (he'd been in the force, before joining the priesthood). In response to the meeting, she does some digging into the issues that the guy raises (as does her daughter), which leads them to investigate old school Roman religions, and their impacts on local people, leading to some grisly results.
It's a good book, just somewhat depress depressing. We find out who the murderer is, sure, but it's not exactly the happiest of endings, so if you were to read the series I wouldn't necessarily start here.