This is the sequel to The Killing Way, Hays's previous work of Arthurian mystery--the genre kind of mystery, not the religious kind of mystery, although religious discussion certainly plays a large role in this book considering how essentially secular its hero is. This is the continuation of the story of Malgwyn, King Arthur's one-armed detective. If you are a sucker for Arthurians, these books will be essential, because they have a fresh take on the original cast and a different plot. If you like historical mysteries from pre-modern evidentiary standards, this may also scratch your itch. I am a sucker for heresies, and this one has Pelagians in it. Pelagians interest me. I had not connected St. Patrick with the era of Pelagius and his heresy before.
I'm not sure how well the very ending--after the mystery plot is resolved, and also a tiny bit of the resolution--will work if you haven't read The Killing Way. If you have, I think it's grounded in the previous work; if you hadn't, it sort of comes out of nowhere, a "wait, what? who? he's mentioned these people, but how did we get here from there?" So this is one of the mystery series I'd advise reading in order if you're going to read them. Luckily it has only two volumes, so this should not be too great a burden.