A melancholy story about a school/sanitarium for young people who have visited fantasy worlds and now find themselves unable to return to the Other World they consider their real home. Their parents believe them to be psychologically traumatized runaways or kidnapping victims, but all any of them want is to find a doorway back. As one teacher explains, you are far more likely to be struck by lightening than to find a doorway, and the chance of finding a second doorway is beyond improbable.
If you have ever read the Narnia books you know the saddest moments were when Aslan would tell a child they could no longer return to Narnia. With the exception of Neil Gaiman's short story "The Problem of Susan," I do not know of any story that has explored the psychological impact of having lived in a world of magic only to be kicked out and the door locked behind you.
About midway of the novel the plot takes a startling turn that I was not expecting. Looking at the author's body of work, it is not surprising she chose the direction she did, but I wish it had stayed more psychological and gone into more detail about the invented cosmology of the other worlds and less into horror.