A young London police officer assigned to guard a crime scene finds himself taking a statement from a ghost. This starts a chain of events that lead him to join a secret adjunct to the London police that deal with matters related to magic and supernatural creatures. The plot involves a series of violent crimes in the Theatre District and a conflict between river spirits. It is all kind of Harry Potter meets the X-Files, but not really as sharp as either one.
I picked this book up because Ben Aaronovitch was responsible for one of my favorite old Doctor Who episodes (Remembrance of the Daleks - 1988). Like a Doctor Who episode this story has some fantastical elements and strange creatures, some danger and running for your life, a puzzle that takes the characters too long to figure out, and it is entertaining if you don't give it too much thought.