An American idiot turns up dead in a London Underground tunnel. The body was stabbed rather than trained, so in comes the murder squad, and Peter Grant smells magic. This leads to -- well, by the end of the book you'll get a lot of sewer-trekking, a spray of gunfire, a trip to the art museum, and the world's most polite race of humanoid underground dwellers; but the mystery is solved by plenty of knocking on doors and talking to suspects. And writing out reports. That's what "procedural" means, and when an author can make that entertaining and throw in a bit of magic, I'm in.
We get just a hint of progress on the series' nominal arc-plot, involving a masked evil wizard, and a bit more on Peter's evolving relationships with his various cohorts and sidekicks. None of this seems to be evolving in a great hurry, so the author is presumably in for the long haul. Fine by me, except that I ran through the three extant books in three months, and now I have to wait like everybody else.