I haven't read the book before this one in the Rivers of London series but I had to buy this when it came out.

It's great how Aaronovitch has kept his character, Peter Grant, maturing throughout the series. Long gone is the diffident Cockney lad who hardly knew how to manufacture a were light without burning the place down. He still works hard at his magic spells but he's up to doing fourth order spells on the fly now. And he's about to become a father!

As this book starts out we see Peter going for a job interview as a security person at a technology company having been dismissed from the London police force. Fortunately, it only takes a couple of chapters to learn that Peter is undercover. It all started with a theft of a piece of music for a fairground steam organ that Peter had investigated. The music was about Ada Lovelace, daughter of Lord Byron and perhaps one of the first computer programmers. The music was meant for an engine that might have been invented by Charles Babbage. It is now suspected that Terrence Skinner, owner of the Serious Cybernetics Company, has his hands on that machine and is holding it in his company's building in London. So, Peter is tasked to find out if it exists and what Skinner might be planning to do with it. As Peter gets deeper into the geek culture he finds that other parties may be after the same thing and he believes that Skinner may be planning to develop a genuine working artificial intelligence. If he succeeds then what would that mean for mankind and the planet? Peter and the rest of the crew don't really want to find out which means finding the mechanism and taking it out of Skinner's control.

Now that Peter is living with Beverley, River Goddess, and soon to be mother of his child, he has other responsibilities. He seems to be able to handle them, along with his police responsibilities.