The Hanging Tree is the sixth novel in the PC Peter Grant series by Ben Aaronovitch. I thought it was the last in the series, but luckily I was wrong.
Peter is back in London and finds himself assigned to a case of accidental overdose. There doesn’t seem to be anything ‘Folly’ related about the case, until it becomes clear one of the witnesses is the daughter of Lady Ty. Goddess of River Tyburn. Sister of his girlfriend Beverly Brook. And she’s going to cash in that favour he owes her for saving his life when he was buried alive.