Duel Nature by John Conroe

Book 4 of the Demon Accords.

Chris and Tanya have been assigned as Coven Rovers, travelling the country and troubleshooting the supernatural community. It's a job that's part cop and part auditor, one that should be long periods of boredom mixed with small parts of action. But this is Chris and Tanya, so a sudden assignment to check on a rogue vampire leads to a run-in with a hostile master vampire, a hit squad, monsters straight out of Native American folklore, black ops teams, and, worst of all, vampire politics. Now more than ever, Chris must control his dark side, as the alternative is death.