A friend of mine recommended the movie to me, which I was surprised to learn is an anime movie that was nominated for an Academy Award in 2005 (lost to Wallace and Grommet, which I find creepy but my kids love). I have yet to see the movie, but I found the sequel to this book in the library during a fly-by and was thus reminded to check out the book. Sorry, long intro.
The basic premise is that Sophie gets turned into an old woman by a witch and goes out to seek her fortune and try to get her own figure back. Along the way she strikes a bargain with a fire demon and becomes housekeeper to the wizard Howl. It was a pretty good story. Not fantastic, but I did enjoy it. There was a little too much description of the scenery for my taste.
The sequel, written some 22 years later, I found much more compelling. It's called House of Many Ways. It has a different main character and a different setting, but Sophie and Howl and the fire demon get wrapped up in the plot, as well. The author employs a very imaginative use of magic (bending time and space).
Apparently there's another one called Castle in the Air. I think they all stand on their own and are only sequels in that some of the characters overlap.