Is four years a long or a short time? When reading books in a series I feel that four years could be slightly too long time between reading two consecutive books, but that is the time I spent before reading Whispers Under Ground by Ben Aaronovich after reading the previous books in the series (Rivers of London and Moon over Soho).
The story itself starts when a dead body is found at Baker Street tube station. It turns out to have been an American art student, and some circumstances around the case suggest that there may be some kind of magic activities involved. The last fact means that Peter Grant (the Metropolitan Police unit for supernatural cases) gets to take part in the investigation, while the first fact means that FBI is also present. This leads to some interesting cultural clashes, and also some tube and sewer system spelunking. In addition, there are some unfinished business from previous books that are continuing (and not concluded).
I enjoy the setting in central London, which is not unexpected. I also enjoy understanding some of the references that I probably would not even have noticed if I had not lived in the area for four years. For instance, I assume it is not by accident that the final chapter, in a book where all chapters have names after tube stations, is called Mornington Crescent… The descriptions of how magic works and how policework is done are also interesting and detailed. The main problem for me was that I did not find the actual main case in the book particularly interesting. It also took some time for me to refresh my memory of the increasing number of characters appearing in the story. This will probably not stop me from continue following the stories about Peter Grant, because they have a nice relaxed atmosphere and are written in such a good mood that the criminal stories are almost irrelevent in the overall experience.