The book wasn't merely abridged, it was thrown out entirely after a certain point! The novel was written by Diana Wynne Jones, and there's a sequel of sorts, "Castle in the Air" (not to be confused with Miyazaki's "Castle in the Sky"). The SO found Howl's Moving Castle pretty easily in the local book store, so if anyone's interested they can probably find it.
I think it's best to judge them as two separate works, rather than a book and the movie version (as in The Last Unicorn, 2001: A Space Odyssey, etc), because the plots diverge so radically within about 15 or 20 minutes of the movie.
But I liked both of them. In general, I like Miyazaki's work. I recommend the movie, and the book, but as I've said don't think of them as the same story.
P.S. As someone said, yes, the very end is kind of a deus ex, or "where'd that guy come from?" situation. There's Nothing to See Here. Move Along.
TWK