by Ben Aaronovich. Third in his "Rivers of London" series.
When your unit of the Metropolitan Police Department consists of one chief inspector and yourself, a lowly constable, it is not the inspector who has to answer the phone in the middle of the night. Thus PC Peter Grant, still the only official apprentice of the last wizard in the Metropolitan Police, finds himself called out to the site of a murder in the Baker Street Underground station, just to check for anything weird about it. Of course, there is some weirdness, and the fact that the victim turns out to be the son of an American Senator who gets an FBI agent assigned as an observer is the least weird part.
I really liked the first two books in this series, and the third installment doesn't disappoint. It's an interesting take on urban fantasy, being at least as much police procedural, and the police procedures are extremely authentic; you don't see a lot of urban fantasy protagonists wondering how to phrase an official report so that it doesn't come right out and say "I detected the presence of the murder weapon by the magical residue it left in the tunnel." Once again, the voice of Peter as narrator is frequently so funny I had to stop and read out-of-context bits to anyone who happened to be nearby. A lot of these had to do with shout-outs to other fantasy, like Peter being told that Gandalf could probably drink him under the table; and I have some thoughts about the provenance of a beer referred to only as "Mac's," which we're told comes from a microbrewery in the States.
There are a couple of plot threads with no beginning or end in this novel--I would definitely recommend starting with the first one; the author has said that what he has in mind for this series is something like Ed McBain's 87th Precinct books (of which there were 55 in all, according to Fantastic Fiction), so there may end up being a lot to keep track of. I'm already looking forward to the next book, Broken Homes.