The Time of the Ghost. Luckily I found a used copy a few months ago and I saved it for the RIP Challenge. It turned out to be even more appropriate than I thought it would be.
A ghost appears in a field, outside of what appears to be a boarding school, but she has no memory of who she is or how she is connected to the place. As she visits the school, she starts having feelings of familiarity even though she isn't remembering much. She thinks the three girls living in the back of the boarding school may be her sisters but she's not sure. The one thing she is sure of is that danger is coming from an unexpected place (and time) and she's the only one who can stop it.
I don't have a copy with this cover but I think that it represents the story better than the cover I have. There's an ancient evil force that scared the bejeezus out of me. The story was a bit patchy and uneven and it was hard to tell where it was going. Also the characters were strange and somewhat violent (especially the parents) and it made it hard to connect with. This is definitely a book for Diana Wynne Jones fans and not a good place to start with her catalogue. That said, I enjoyed it and thought some of the twists were well done.
Keeping the lights on,
We bought our own copy of this book.