A ghost can't remember which one of four sisters she is, or how she died. Time and space are woven into a landscape, and things and people get stuck in between. A star is also a celestial being before being turned into a dog. A girl discovers that her aunts and uncles are the Pleiades and various Greek Gods. A world just like ours, except that everyone takes for granted that witches ought to be burnt at the stake.
What I loved about Diana Wynne Jones' books was their sheer inventiveness. She had a way of looking at the world that no one else had, and way of imagining magic that owed nothing to anyone who came before her. Magic overlaid and intertwined with the real world in her novels, but they were also grounded in real emotional situations -- the feuding parents of The Lives of Christopher Chant and Aunt Maria, school rivalries in Witch Week. Above all she could take an abstract concept -- time, for instance, or mythology -- and make it a real-world object without ever seeming gimmicky or heavy-handed.
My favorite Diana Wynne Jones novel is The Time of the Ghost, which is also one of her earliest. It begins with a ghost who is unsure who she is or what happened to her. Observing her sisters and family, she begins to piece the story together, until about half-way through the book, when the narrative jumps forward in time and the reader realizes that nothing is what it appears to be. The sisters in the book, and their situation -- living in a boys' school, neglected by their parents -- were based on Jones' own childhood, which adds to the darkness of a book that is already concerned with death and witchcraft. Jones would go on, in her prime, to write books that were much more inventive than The Time of the Ghost. But I've rarely read a YA book with such emotional depth. If you've never read Diana Wynne Jones, go look for the The Time of the Ghost. Then read Dogsbody, and then the Chrestomanci books, starting with The Lives of Christopher Chant. And then you should still have a couple dozen more books to read. You'll enjoy every one of them.