Movie Howl is a soulful and quiet. Book Howl is a drama queen and Causing Problems and has a long string of jilted exes and couldn't shut up if you paid him.

Sophie and Howl drive each other up the wall at the beginning and it's really funny. Sophie and Howl are (despite themselves) very much in love by the end and they still drive each other up the wall and it's even funnier.

In the movie, Howl has been ordered by the king to participate in The War, and Howl is avoiding it because he is a brave conscientious objector. In the book, Howl has been ordered by the king to rescue his lost brother from the Witch of the Wastes, and Howl is avoiding it by any means necessary because he is a cowardly weasel who wants to stay as far from the Witch as possible.

In the movie, the Witch cursed Sophie because she was jealous about Howl speaking to Sophie for five minutes. In the book, the Witch cursed Sophie because Sophie had been doing surprisingly powerful magic for years without knowing it and it was actually starting to cut into the Witch's plans. (Sophie does not discover any of this until nearly the end of the book, but the reader can start to pick it up much earlier and the way Sophie's magic works is pretty darn cool.)

In the movie, there's a rumor that Howl eats the hearts of maidens, but this is implied to be nothing but nasty fearmongering. In the book, there's a rumor that Howl eats the hearts of maidens because Howl started the rumor so people would stop asking him to do wizard junk all the time.

The book lightly parodies a couple of tropes from Western fairy tales. In particular Sophie has internalized that, as the eldest of three sisters, her "destiny" is to fail so that her younger sisters will look cooler when they succeed, which is why she's so resigned to the hat shop at the beginning. (Sidebar: Sophie's sisters come up much more in the book and they're great.) There's also a really funny bit where Sophie attempts to operate a pair of seven-league boots.

you have to be kinder to people who are slow processors.

you have to be kinder to people who don't understand your jokes.

you have to be kinder to people who forget important dates.

you have to be kinder to people with cognitive decline.

you have to be kinder to people who were always this way, too.