One quick note about the commentary on Howl's Moving Castle: A bunch of people in the supplemental materials on the DVD say things like "Wow, the idea of a girl being turned into an old woman--that Miyazaki guy is amazingly inventive!" Or "Wow, the idea of a door opening in different places--only Miyazaki could have come up with something like that."
The problem with these comments, of course, is that at least half the things people are commenting on came from the original book.
More generally, I was a little surprised that nobody mentioned the book in any of that making-of material. I wouldn't expect them to spend a lot of time discussing it, but I would've thought someone might say, even in passing, "It's based on this book by Diana Wynne Jones." (It's possible that someone said this and I just missed it, though.)
. . . In case anyone's curious, here's a Telegraph article that gives a little information (near the end of the article) about DWJ's thoughts about the movie (and indicates indirectly that she wasn't involved in making it at all, beyond signing over the rights).