What Abigail Did That Summer: A Rivers of London Novella by Ben Aaronovitch

Ghost hunter, fox whisperer, troublemaker.

It is the summer of 2013 and Abigail Kamara has been left to her own devices. This might, by those who know her, be considered a mistake. While her cousin, police constable and apprentice wizard Peter Grant, is off in the sticks chasing unicorns Abigail is chasing her own mystery. Teenagers around Hampstead Heath have been going missing but before the police can get fully engaged the teens return home—unharmed but vague about where they've been.

Aided only by her new friend Simon, her knowledge that magic is real and a posse of talking foxes that think they're spies, Abigail must venture into the wilds of Hampstead to discover who is luring the teenagers and more importantly—why?