This series is brilliant!
The Hanging Tree is definitely one of the best in the Grant/Aaronovitch pantheon. There again, so have most of the previous books in the series and the odds are the next one probably will be too.
Aaronovitch deftly balances urban fantasy with the police procedural. As for Grant, he's a wonderful blend of laconic copper and, methodically researching how magic works, full-on nerd.
Peter Grant is back, and now in handy-to-read-on-the-train novella format.
The Rivers of London series is an ever-evolving delight.