The Rivers of London by Ben Aaronovitch

I do like a book involving magic and I was immediately attracted to this book when I read a review of it in SFX magazine.
Peter Grant is a London policeman who is just coming to the end of his probationary period. He is not a particularly gifted policeman but in the course of a murder investigation he meets Chief Inspector Nightingale. It turns out that Nightingale is the last wizard in England and that Grant shows an aptitude for magic. So Grant joins Nightingale's department (there are only the two of them in the entire department) and becomes a trainee wizard.
The murder that brought Nightingale and Grant together has supernatural elements to it. People who have no motive and no previous history of violence are suddenly murdering each other. Nightingale believes that they are being possessed by an unquiet spirit. Grant finds himself questioning ghosts and performing arcane rituals in order to discover the identity of the spirit. He also finds himself mediating in a land (or water) dispute between Mother and Father Thames.
I really enjoyed this book. Grant is plunged into a world he didn't know existed and he's having to learn on the job. The story is written in the first person and Grant's voice is alternately amazed, confused and scared. Nightingale is mysterious and we don't learn much about his past, but it does seem as though his past might be longer than his appearance suggests. One thing that would've helped me is a map of London. The city is important in the story and I would've liked to trace where the action was taking place.