Midnight Riot by Ben Aaronovitch
Midnight Riot was so fun, in the way that murder, magic, ghosts, and British slang can sometimes be. Constable Peter Grant is on probation and about to be assigned a post where his days will be filled with paperwork. It is only by (bad?) luck, that he stumbles on a murder scene and meets an eyewitness who happens to be a ghost, a ghost he can see and talk with. Peter ends up being assigned to assist Detective Chief Inspector Thomas Nightingale, who runs the Special Operations Unit, responsible for investigating the "unusual" in London. In this first in the series, the two are dealing with a spike of unusual murders in the city while also trying to broker a deal between the gods and goddess of the Rivers of the Thames.
Peter, from whose viewpoint we see the story, is a fabulous character. He has is self-deprecating and firmly rooted in the modern world. He also has an interesting family...