Rivers of London by Ben Aaronovitch

This is London as you've never seen it before. A city of ancient secrets, which is haunted by its past. A city where you are never far from magic. A city Peter Grant will help you discover:

My name is Peter Grant, and I used to be a probationary constable in that mighty army for justice known to all right-thinking people as the Metropolitan Police Service, and to everyone else as the Filth.

My story really begins when I tried to take a witness statement from a man who was already dead...