So many differences between the book and the movie!
I watched this years ago and didn't remember it much. Then, just recently, I read the book and couldn't put it down! I decided to re-watch the movie to see if I would appreciate it more. Probably a big mistake.
The film is interesting, but they changed so many of the characters, plot, etc. I guess they had to make it more "dramatic." Markle (Michael in the book) isn't supposed to be a child, and Sophie has two sisters, one of which is his girlfriend. Howl is obsessed with his appearance in the book because he's always wooing women, trying to find one that will love him perfectly and break his curse. The wizard Sulliman is both male and not evil. The Witch of the Waste is still the main villain, and wants to use Howl, Sulliman and the prince to make a perfect puppet to control, not because she wants Howl's love. The war isn't happening directly in the book, or is far away. The King's quest for Howl is to find his brother so his brother can help w/ that, not to have Howl and the other wizards fight. The dog wasn't a spy or in love with Sophie. He and the scarecrow were cursed by the Witch of the Waste. So many more things!
Anyway, if you are confused or have questions about certain things in the movie, the book MIGHT answer your questions. Otherwise I suggest reading it simply for the awesome story.
I would love to see a true adaptation of this book. The movie as a stand-alone piece is still interesting, but don't expect it to be like the book.