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.
Ben Aaronovitch is a master of metropolitan magical mayhem and the action is delightful. This is a tight thriller that is compelling and buckets of fun.