This book is a difficult read. It is not easy sailing, though the writing is superb. This book is unconventional to the extreme. There are two confusing points in the book, one of which is the ghost's identity (which sister is it?), and the other is that fact that the ghost is NOT a ghost of a dead person. But you can figure out the ghost's identity fairly quickly if you pick up several obvious clues that many seem to miss, and this book is one of my favorites.
I won't go into the plot--other's, such as the publishers, have done it already--save to say that it is not gory or freakshly disturbing. If you are zealosuly religions, then you may not like this book, as it does have a "dark, old, female something" (a goddess-like presence) and severe neglect from the parents regarding the four sister-protagonists. One of the sisters goes missing, and the parents doen't even notice after several days, even though the mother comes in to say good-night and the father throws a rage at them later, even going through all four names without noticing before leaving.
This book is more like a window into a at-once familiar and fantastical world than a science-fiction/fantasy novel, a world where things that most people go through in childhood (such as a play-sceance using scrabble-letters or a belief that there is a ghost in the house) do not collapse into disbelief with time but are confirmed in a subtle, definite way. THAT is the whole of the 'horror and occult' in this book.
The characters are exactly drawn. Perhaps others do not know people as unusual or interesting as the four Melford sisters, but they are the sort of people who are at the edge of the population, who turn into famous artists, writers, musicians--the sort of people who go into history books (the sort of people who are very unusual). They are disinctly portrayed, and you can feel that you would recognize them on the street after reading the book. The personalities are very vibrant, and the entire book is a pleasurable read. You may want to reread this book later, because it is one of those books that can be reread many times; not open and direct, but secretive, mysterious, and very well-woven. Five stars on this one. Diana Wynne Jones has done an excellent job and has not been afraid to step out of the borders of conventional fiction writing to turn out a novel of striking originality.