Fair Game by Patricia Briggs

The Marrock's enforcer, Charles, has begun to experience some serious problems because of the frequency with which he is being dispatched to execute werewolves who have committed crimes against normal humans since the weres have outed themselves. His wife, and pack Omega, is distraught, and appeals to Bran to do something about it. When a request from the FBI for help on a serial killer case comes to him, he hopes that sending Charles and Anna as consultants will at the very least distract his enforcer long enough for him to get a handle on his ghosts.

They journey to the East Coast and meet with representatives of the FBI, Homeland Security and a secretive government group known as Cantrip. They are briefed in on the killer's victims and methods, and soon suspect that he or she may have some supernatural aid, as werewolves are also being killed, and the most recent person kidnapped has Fae blood. Charles and Anna enlist the aid of the local pack, and consult with two of its most powerful witches to track the killer, who appears to be using black ritual magic to drain power from his victims.

A great deal of this novel is spent on Charles and Anna's relationship, their mystical bonds, and how Charles' guilt and perhaps even some actual haunting by the spirits of those he has slain is affecting him and the two of them. It also makes some progress in the overall plotting of Briggs' books in advancing the cooperation and communication between the human government agencies and the pack, and in a twist at the end introduces a new crisis between humans and Fae.

Fun reading, not terribly deep, with plenty of good action when the killer and minions are caught and battled.