Oh yes, I read it before I knew of the movie because it just so happens that Diana Wynne Jones is one of my favorite authors ever! If you haven't read "The Darklord of Derkholm" (satirical and hilarious) or "Deep Secret," you must. HMC wasn't one of her best books in my opinion, but when I heard Miyazaki was doing a film version, my interest was piqued. Whaddya know, it's now one of my favorite movies! Howl's such an endearing character. I've been meaning to do some fan art (not that I have the time), but not anime style- I despise anime. Except for HMC, of course.
Whoo, that was a long-winded reply.