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.