The October Man (Rivers of London, #7.5) by Ben Aaronovitch

With this long new novella, bestselling author Ben Aaronovitch has crafted yet another wickedly funny and surprisingly affecting chapter in his beloved Rivers of London series.

If you thought magic was confined to one country—think again.

Trier: famous for wine, Romans and being Germany's oldest city.

When a man is found dead with his body impossibly covered in a fungal rot, the local authorities know they are out of their depth. But fortunately this is Germany, where there are procedures for everything.

Enter Tobias Winter, an investigator for the Abteilung KDA, the branch of the German Federal Criminal Police which handles the supernatural. His aim is to get in, deal with the problem, and get out with the minimum of fuss, personal danger and paperwork.

Together with frighteningly enthusiastic local cop, Vanessa Sommer, he quickly links the first victim to a group of ordinary middle aged men whose novel approach to their mid-life crisis may have reawakened a bloody conflict from a previous century.

As the rot spreads, literally, and the suspect list extends to people born before Frederick the Great, Tobias and Vanessa will need to find allies in some unexpected places.

And to solve the case they’ll have to unearth the secret magical history of a city that goes back two thousand years.