This is the third book in the series, and it continues to amaze me how Good and Wholesome it is. The writing is simply delightful, and Seanan McGuire does a lot with her concept of secondary worlds. I simply loved, from the beginning, the idea that "misfit" children, who don't really belong in our world, get to find one they truly belong into. Maybe because I was one such kid who dreamt to escape to another dimension. 
But this novel takes it one step further, walking us through different worlds and different experiences, from the trans Goblin Prince to the hijabi Baker, the Mexican-American kid who makes music for skeletons or the fat Mermaid who's the main character of this story, they're all really about belonging, and I truly enjoy the fact that these kids are kind to each other, and even if their worlds are (temporarily?) lost to them they find belonging in their shared differences. 
This story comes with a huge trigger warning for fatphobia, which is at the centre of Cora's experience, but at the same time it's not about her being the victim of bullying, it's more about her power as a fat, athletic girl, and her finding a place in the world where her appearance is an advantage, and even using others' fatphobia against them. It was really empowering to read.