I had to read the book. And I'm astonished at how much better the movie was - the script is tighter and with fewer distracting elements. I never thought I'd say a movie was better than a book for anything after Jurassic Park. The character sketches add so much to the story in the anime.
The story is about the same, much more elaborate and explanatory being in words. Diana Wynne Jones ties the whole story together beautifully in the end - most authors leave threads hanging, even the movie leaves you with questions, but her book pulls it all together in less than a chapter.