Diana Wynne Jones' Reflections On the Magic of Writing includes a transcript of literary critic Colin Burrow's BBC essay, Fantasies for Children, which you can listen to. Burrow just happens to be Wynne Jones' son.
Burrow says that Jones fused the ordinary and the magical, which may be why I've liked what I've seen of her work. I can only take so much magic. He also says that Fire and Hemlock is her best book. What!? Not Chrestomanci?
Burrow talks about Jones' feelings about her childhood and how they impact her writing. If you read Reflections On the Magic of Writing, you hear a lot about that from her, too.
August 16, 2014: Diana Wynne Jones should be referred to as "Jones," as corrected above, and not as "Wynne Jones."