This one is more a story about the magical world of London and England and allows for later expansion of the world, it turns out that there are a few schools of magic in the world. The adventure all starts with Lady Tyburn asking Peter for a favour and to keep the favour quiet, however Peter knows better than to try to hide things and tells everyone above him so that they can be prepared for it. An overdose. At a secure building with one of Lady Ty's daughters involved. One of the others involved is a magic user too and through this teen Peter finds out that there's another magic tradition and has to face up to the fact that there was a lot of sexism in his order.
It features some lesbian characters and a muslim fellow police person who wears a hijab and is slowly creeping towards being a member of the Folly. Peter is determined to bring the Folly into the 20th Century and seems to be determined to write up everything to help others. He believes in information and less secrecy but understands that some things people don't want to know and occasionally some information can kill or do more harm than ignorance.
I love this series and when I read I often have to share paragraphs with my husband, Peter has such a wry tone and cynical view, he feels like someone who has worked as a copper for long enough for some of his inocence worn off. He knows the system and knows how to use it. Other innocences are being worn away too.