The Harry Potter reference comes because of the magic in the story. The idea of a policeman having the ability to talk to ghosts and as a result the chance to become a wizard's apprentice forms a large part of the story.
It gives Ben Aaronovitch a chance to introduce vampires, creepy ghosts that inhabit the bodies of innocent Londoners and an insight into the training of an apprentice wizard with lights formed by the mind one of the main speels that gets worked on.
As well as having fun introducing the idea that the modern streets of London could be populated by vampires and ghosts there is also a lot of mileage to be had in the main character Peter Grant because he is a young officer in the Met who has a mixture of wide-eyed wonder and cunning that makes the whole wizard's apprentice premise much more believable.
But for me the other main character in the book isn't Detective Chief Inspector Thomas Nightingale, the last English wizard who lives in the Folly, an old building protected by magic, but it is London itself.
Set around Covent Garden and involving moments when the story of the City goes right back to its formation this is a tour through a particular area of the capital. Plenty of history is thrown in and facts about the police, the theatre and the area but it is usually done with a light touch. As a result you are learning about the story of London and the idea of wizards being employed by the Metropolitan Police feels as if it could legitimately be part of that tale.
The story is delivered with a mix of humour and when needed graphic horror as Grant gets to grips with the ghost of a frustrated actor. But there are other threads here that you know will be picked up in other books, including the references in the title, to the Rivers of London. It comes as no surprise to find out there is a Father Thames with his brood of children named after Thames tributaries. But there is also Mother Thames and the battle between the two runs as a backdrop throughout the book.
So on one level Peter Grant is Harry Potter in the Police. But on another this is so much more because the world he inhabits is not a fictional land of Hogwarts but London. The same London that is waiting for anyone to pop up at Covent Garden tube and stroll through. The ability to use the capital as a backdrop, provider of mystery and history is what makes this book stand out.
