Midnight Riot

Midnight Riot is by Ben Aaronovitch. The first book in the fantasy Rivers of London series. Peter Grant is a Probationary Constable in the London police force. He is about to be assigned to a permanent division; then he discovers he can talk to ghosts. He is then assigned to work with Detective Chief Inspector Thomas Nightingale whose unit investigates cases with a supernatural bent. Now a bizarre series of vicious murders are sweeping London and Peter finds himself involved with gods, goddesses, and all manor of strangeness. There is humor in this series as well as violence. 

Ratings: 11th grade - 8 out of 10 - V (violence) - P (profanity) - AC (some mature thematic content).