Dead Man's Hand - James J. Butcher

On the streets of Boston, the world is divided into the ordinary Usuals and the paranormal Unorthodox. And in the Department of Unorthodox Affairs, the Auditors are the magical elite, government-sanctioned witches with spells at their command and all the power and prestige that come with it. Grimshaw Griswald Grimsby is... not one of those witches. After flunking out of the Auditor training program and being dismissed as 'not Department material,' Grimsby tried to resign himself to life as a mediocre witch. But he can't help hoping he'll somehow, someway, get another chance to prove his skill. That opportunity comes with a price when his former mentor, aka the most dangerous witch alive, is murdered down the street from where Grimsby works and he is the Auditors' number one suspect. Proving his innocence will require more than a little legwork, and after forming a strange alliance with the retired legend known as the Huntsman and a mysterious being from the Elsewhere, Grimsby is abruptly thrown into a life of adventure, whether he wants it or not. Now all he has to do is find the real killer, avoid the Auditors on his trail, and, most importantly, stay alive.