The Diabolist, by Layton Green

I accidentally entered Mr. Green's series late in the game: this is the third novel featuring the main characters. They are Viktor Radek, a professor of occult who also investigates odd, para-normal situations, and his partner Dominic Grey, who is mostly the muscle and on the ground investigator. The book is good although the action gets periodically interrupted for detailed information about organized religion, satanists and the occult.

The Diabolist (The Dominic Grey Series)