What Abigail Did That Summer - A Rivers Of London Novella

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?