This book is part of the Magpie Lord universe. It is the least connected of the series and really could be read on its own. It features two fun cameos and the justiciary including Stephen Day, but sadly no Lord Crane.
Crispin is a warlock by accident, having been trained in blood magic by a man that wanted to wear his skin. He is struggling with using magic in the approved ways and feels like a failure. The only joy in his life comes from his lover Ned. Ned is not a magician. He is a street paper collector and seller, uneducated, and black in Victorian England, so he has enough problems without magic and is a little icked out that his boyfriend carries a pen made out of a pinky bone. But someone is using magic to kill in the London slums and no one else seems to make the connection or care, so Ned starts an investigation of his own.
This story can only loosely be called a romance, its focus is on the magic and the mystery. I did not get a real sense of why the men liked each other since they were already in a struggling relationship when the book starts. I understood what was bothering them and why they might break up, but did not feel any strong sense of what would keep them together. But by the end I was rooting for them to be happy because I did like both men. I enjoyed this book and it was a quick read, but it is not my favorite in the Magpie world.
Edited to add: so I just saw that there is a book Queer Trade that I think tells the story of these two getting together.