I don't want to spoil the plot of this book, because the first chapter is so atmospheric and Jones hides the problem of her heroine so cleverly. In fact, the first part of the book is very atmospheric and very clever; it's only in the second part that things start to unravel, because you have a POV twist which doesn't quite work, and at the end of the story you're left scratching your head wondering what happened. Of course, those who liked it will go back and read again.
Unlike 'Fire and Hemlock', the mythology in 'The Time of the Ghost' is not so strong - actually, I wish she'd expanded upon this, as it only appears in the second part of the story. It would have perhaps added a deeper layer to the story, which ends up being rather like the film 'The Craft' (although this book was written before that).
Oh, and if you don't believe that four girls could be this neglected by their own parents, Diane Wynne Jones based the girls' situation on her *own childhood* with her younger sister, and actually left out some incidents because she thought that people wouldn't believe them! (Like the youngest sister, she did her hair in knots and waited to see how long it would take for her mother to notice - apparently, it took her mother two weeks.)
It's not her best, but it's still a brilliant read!