Book One: 

Mark's job seems pretty dull, working as an investigator into crimes committed against Church property, theft of holy objects, vandalism; nothing terribly exciting but he does get to travel the world.

That's just the window dressing. Mark does work for the Church, but as an elite member of a thousand year old secret society that hunts down devils, demons, and other evils. His job is not just to find them, but to remove, dispel, or kill them: He's on the front lines of the secret ongoing war between Heaven and Hell. However as wars go, it has been a fairly easy one for the last few decades, with nothing seriously evil having been summoned since the last world war.

But all of that is about to change, and the question for Mark may not be can he survive, but can he survive long enough.

Book Two:

With having to leave the Commission for a year while things cool off politically, the Church has loaned him out to the FBI, who have been trying for years now to get an experienced monster and demon expert to help train and educate the agents there on just what they will be facing.

Mark doesn't mind the new assignment, being closer to home it will be easier to visit his family, and the agents all seem nice enough. However, while Mark knows how to deal with devils, demons, and even the nastier monsters out there, he doesn't know anything about how to deal with a sociopath werewolf who has gone full psycho and started to murder co-eds. 

That's more of a "human" problem, after all.

Book Three:

Being on loan to the FBI has its advantages, and for the first time in far too long, Mark is taking a few weeks off with his family. But he's barely gotten started when he finds his brother unwittingly entangled in a mess that can be traced directly back to an attempt on Mark's life some months ago.