I personally have never read anything by her, though I own a few of her books and have a fair few more on my to-read list. I know a few people who push her work hard, often listing her as one of their all-time favorite authors, and just as often, Howl's Moving Castle and Dogsbody as some of their all-time favorite books.
Many of her books have been reissued in recent times, and she has a new book scheduled to be released this summer, Earwig and the Witch.
She is the author of
and many, many more.
For a lovely tribute to this beloved children's author, see The Huffington Post's "Diana Wynne Jones: An Appreciation"