Unputdownable is quite an apt word for this small book. It's difficult to know how to classify it, because the story doesn't quite fit into the genre of horror. It takes the reader to the edge and shows her the horrors that lie beneath, but stops short of actually terrifying her.
There is a sequel as well - Ben, in the World. The Fifth Child ends with Ben about to leave home and out into the world. The sequel seems to be about how he fares there.