Every Heart a Doorway by Seanan McGuire

There have always been stories of children disappearing into other worlds-- fairylands, wonderlands, netherworlds, and more. But what happens when those children come back? What happens when their families no longer know what to do with them, when all they want is to jump back down those rabbit holes?

If they're lucky, they end up at Eleanor West's Home for Wayward Children, where they can be among others who understand. Nancy, who visited the land of the dead and learned the ways of stillness, is one such lucky girl, but it doesn't feel so lucky when right after she arrives, other students start turning up murdered and dismembered. As the new girl, from an underworld no less, it's not the best first impression.

This is an insanely creative, surprisingly short novel (or novella, technically) with an impressively diverse cast of characters. The worldbuilding is unlike anything I've ever read before. While the "fantasy boarding school" setup might appeal to teens, the gruesome deaths of the children officially makes this an adult read. There are currently four books in the "Wayward Children" series, with more on the way, and a TV show is in the works.