Book #4 

Start with Dirty Magic. When the Magic Enforcement Agency is called to the scene of a dirty magic lab explosion, a body with a bullet wound is found in the ruins. As Detective Kate Prospero and her partner, Special Agent Drew Morales, hunt down the killer, they uncover evidence that a dangerous new coven may be operating in the Cauldron. It's not long before the bodies start piling up, and the heat is on for the team to make an arrest. Solving the murders will require unraveling dangerous alliances between the city's dirty magic covens.