I was looking forward to this so much I pre-ordered it three months in advance (something I'm not wont to do). When it finally arrived I was looking for a particular good moment to sit down with it and read it in one go.
The comics of Rivers of London are a nice addition to the main series, although it doesn't seem they are essential to the overall plot of the series. Nevertheless I've been enjoying them a lot and would certainly recommend them to fans of the series. Black Mould, as the name suggests deals with an infestation of sentient mould and was another good read. The only complaint I had was that the solution felt a bit rushed, but this was not a major problem.