If you'll remember from the first volume, Detective Constable Peter Grant is part of the Metropolitan Police's "Economic and Specialist Crime Unit". It consists of two people - Grant and his mentor, DCI Nightingale. Nightingale is much older than he appears and infinitely more knowledgable: he's a wizard. Grant, due to a combination of inclination and misfortune, is also a wizard - at least, he's trying really hard at it.
In the first volume, Grant is wrapped up in a fairly horrific little mystery that involves dark magic and people's faces falling in. The second volume starts with the same promise: someone out there is doing something nasty (and magical). Grant needs to solve it. This time around it is also more personal. The naughty-maker is offing jazz musicians, and Grant's dad is one of the best in the business. Although he's always been wary of it, Grant finds himself easily absorbed into the jazz scene. He finds friends (and ladyfriends) and indulges himself in a little second-hand fame due to his father's reputation. These new contacts prove valuable when it comes to snooping about in the dark and spicy Soho underworld.
Just to keep things interesting, there's also something out there eating people's anatomy. Eep.
The balance here is between plot and meta-plot. One of the cases above is a fairly transparent whodunnit. I'm not the savviest mystery reader, but I found absolutely no mystery in whoactuallyddunnit. The only detection was trying to suss if Grant was being particularly thick or if he actually knew all along and was stringing us along for extra overtime pay. This case is the book's self-contained plot.
The meta-plot is a much more complicated situation that involves the mysteeeerious origins of the magical tradition, a potential Big Bad (or Big Bads), a massive conspiracy, the misbegotten youth of DCI Nightingale and all sorts of stuff that is in no way resolved during the course of this book. It is fun - spell-fights and secret histories are invariably entertaining - but this isn't a mystery, it is epic fantasy with a hat on.
Ben Aaronovitch continues the successful formula of Rivers of London in bringing to the surface the endearing minutae of a city that he clearly adores. He also demonstrates the "real world" problems of a ritual magician trying to have some sort of "normal" life - never is this more aptly demonstrated than Grant having to run the broadband out of the garage, lest the cabling interfere with his home's magical protections. Grant continues to observe his world(s) in a detached way - a narrative voice that lends itself well to dry humor (and fits less neatly with the book's few over-the-top action scenes). Mr. Aaronovitch is, in short, writing the best contemporary occult detective series on the shelf today, and that's by a substantial margin.
My neuroses stem from concerns about the balance of "occult" and "detective". I hasten to add that I am making a very large mountain out of a very small molehill. Moon Over Soho is VERY good. One of PC Grant's core personality traits is his emphasis in approaching everything - even the supernatural - in a modern and rational way. Despite his wizardry, he is, in fact, the consummate detective.