The Furthest Station, by Ben Aaronovitch

The seventh book in the Rivers of London series is a super short one, more of a novella or a longish short story than a novel. It takes us back to the London Underground, with a series of ghostly encounters reported on the Metropolitan Line. Constable Peter Grant and his teen cousin Abigail must get to the bottom of magical messages that could lead to solving a non-magical crime. No sign of the faceless man or erstwhile colleague Lesley in this brief tale, but the scene is set for Abigail to be properly apprenticed to the magical unit of the Metropolitan Police.