Series: Rivers of London/Peter Grant #5.7 (not sure why it isn't 5.5)
This novella is an excellent little interlude with a mystery involving ghosts on the Underground and showcases many of our favourite characters: Nightingale, Toby, Abigail, Jaget, and Molly. Even Guleed gets a mention although she doesn't make an appearance. We get exposed to a little bit of the history of the Folly and it's just all around good fun.