Stepping into Diana Wynne Jones's world of whiny wizards has been a treat. There's a quirky fractured-fairy tale feel to the whole book that's actually subtle enough not to overwhelm the story with irony; what Tolkien calls "the author's wink at the other adults in the room." The characters and settings function well both as archetypes and as individuals so that the fairy tale feel is preserved right along with all the trappings of a modern psychological novel. For those who were introduced to the story with the film, it is pleasant to find that Miyazaki preserved enough of the original story to make it familiar and intelligible when turning to the novel and yet provided enough changes and omissions to keep the book fresh and interesting in its own right. So far, my appreciation for neither the film nor the book has been diminished, and that's quite a rare thing.
We're only half-way through the book right now, so I'll have to stop there. If the story continues the way it's going currently, however, they'll be nothing but good news to report once its finished. Good luck in the meantime!