Foxglove Summer (Rivers of London 5)

"When two young girls go missing in rural Herefordshire, police constable Peter Grant has to leave London to check that nothing supernatural is involved. It's purely routine, Nightingale thinks he'll be done in less than a day. But Peter's never been one to walk away from someone in trouble, so when nothing overtly magical turns up he volunteers his services to the local police, who need all the help they can get. But because the universe likes a joke as much as the next sadistic megalomaniac, Peter soon comes to realise that dark secrets underlie the picturesque fields and villages of the countryside and there might just be work for Britain's most junior wizard after all."