Nelson & Macilwraith - Moon Murder Mysteries, #1

No one wants a witch for a partner. Time is running out, and the deck is stacked against Professor Lennox "Nox" MacIlwraith, unorthodox expert on the occult, rumored psychic and witch. Six girls have been abducted and now another has been found dead, tied to a tree. The FBI is refusing to see they're connected, but Nox suspects a serial kidnapper and killer connected to an ancient Celtic cult is to blame. Everyone in the FBI hates Agent Grady Nelson.
Nelson's ready to kiss what's left of his career goodbye when he's ordered to work with the crackpot "sketchy professor" on what should be a nuisance case. The investigation turns into a professional minefield, but all Nelson cares about is solving the case before the next full moon. If not, he fears the remaining girls will be sacrificed to a mythical god-king. Rules are broken and lines are crossed as Nelson falls under Nox's spell and begins to suspect his partner might be a real witch. Nox is a little bit witchcraft, Nelson is a little bit Federal Bureau.
Together, they're a wickedly good team, but can they find the missing girls before it's too late? Nox is prepared to sacrifice his own career and his life to save them. He'd also like to save Nelson, but is Nox ready to sacrifice his heart too?