I don't know the answer, but happy to see someone talking about this book. I feel like so many people know the movie and have no clue that it's based on the book, nor how much they changed it. I personally love the book and am happy to see it.
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I was going to start reading - well, actually, I read the first few pages, but it was more like skimming, so I am restarting my progress again. The movie was really nice, I'd say, but it was focused on world-building to the point that I was not able to grasp details about the characters. Like for example, I never knew that Lettie and Fanny Hatter were two different characters.
Spoilers I've read from wiki pages
I also learnt about Howl being a Welsh child from Earth in the 80s, who was summoned to the out-worldly city of Ingary. Then there's also modifications about the story of Prince Justin who was originally the sibling to the current King, but in the movie, he was portrayed as a wizard-prince from a neighboring country. There's also a third sister that we never meet, I think?
Since I've not read the book, I can't be sure if there's more to it. There's so many stuff that's just missing, or outright changed.
Yeah, also a bunch of other details, and the whole plot is way more focused on the war in the movie. In the book it's more of a backdrop. You should give it a read, it's worth it :) I also like her other books!