This is a delightful supernatural detective story, set in London. It's written in the first person, following the viewpoint of Police Constable Peter Grant. I like the way they set up the magic system, but most of the charm is the almost constant stream of sarcastic asides by the narrator.
Midnight Riot is the US title, the original UK title is Rivers of London. This is the first book in the Rivers of London series. The next two books in the series are Moon Over Soho and Whispers Under Ground.
Police Constable Peter Grant is a probationer, looking forward to being given his regular assignment. Because of his rank, he gets stuck watching a crime scene overnight, along with his fellow probationer (and somewhat love interest) Leslie May.
This is where he first sees a ghost.
While he considers himself a fine beat police officer, his superiors (and Leslie) aren't so sure. They think that he gets distracted too easily. He learns, much too his chagrin, that instead of working the street, he's going to end up at a desk, updating case files.
Salvation comes in the form of Detective Chief Inspector Nightingale. Nightingale heads up, and is the sole member, a Special Unit that deals with anything to do with magic or the supernatural. Also, he's a wizard.
As Peter Grant had inadvertently mentioned to Nightingale that he'd seen a ghost, Nightingale decides to take him on as an apprentice. Apprentice wizard PC Grant tries to solve a series of murders, meets and deals with a variety of river gods and goddesses, and learns some magic. Along the way, he manages to shine a little light on things.
There's a bit of gruesome violence, including some involving very young children. There's also some domestic violence and sexual violence. People die, some quite horribly.