The devil's due - Jenna Black

Things aren't going well for exorcist Morgan Kingsley. She's been betrayed by the boyfriend of her dreams, her brother has been repossessed, and she's unable to escape from the hunky demon who shares her body. All Morgan wants is to curl up and avoid reality for a while. But she can't. A woman named Claudia Brewster seeks her services.

During the brief time that Claudia had been on a cruise, her son, Tommy, changed from an invective-spewing member of the anti-demon society God's Wrath to a happy little demon host. Claudia smells a rat - as does Morgan - but all the paperwork looks legal.
That is, until Morgan begins digging up the secrets of Tommy's past and finds herself deep in another deadly conspiracy...