Every Heart a Doorway by Seanan McGuire is a nice little fantasy tale of the place where children go after going through a Door to Another Place (Fairylands, for example), and then coming back. It’s short, and sweet, and sad, and gruesome. It’s about real children, not a list of characters from other books, either. The gruesome part is the murder mystery half of the book, but that’s really only window dressing for the exploration of the characters and worlds. It’s a bit short, and I’d have liked to see more exploration of both the children and their worlds – but I guess that’s what the sequel is there for.
If you just want to read more general portal fantasy, without this level of meta, look at things like The Girl Who Circumnavigated Fairyland in a Ship of Her Own Making