Some poor fool left a copy of Howl's Moving Castle lying on the table, so I stole it and read it. It's very rare that a movie is better than the book, but I seriously loved the movie and I wanted to check it out!
I can't tell you which is better, because they're so different, but I can talk bout how they're different, which to me is a neat topic.
Like I said, the movie and book are very different. The stories only vaguely resemble each other, with most of the major characters being at least recognizable, and Sophie's curse and Howl's deal with Calcifer being almost identical. Since those two drive the plot, you can at least tell the book is the source of the movie. This isn't like, say, Who Censored Roger Rabbit, where the movie (thank goodness) throws out everything but a few character names and starts over. It isn't like Jackson's Hobbit movies or the Rescuers (or Peter Pan, or Mary Poppins, or... well, anyway, thanks, Disney) where the movie is the opposite of the author's intent.
Instead, they're... different. What's the same and what's different? Well, that's what interests me. The book and movie have very similar beginnings, but as they go along the story diverges more and more. Most of the movie's conflicts and events, like Howl's opposition to the war or Suliman trying to trap howl, aren't in the book. The book focuses closely on Howl's conflict with the Witch of the Waste, and with Sophie's personality and her status as oldest of three sisters in a fairy-tale world. These issues barely get walk-on parts in the movie.
Miyazaki blatantly uses the movie to push his own moral messages that aren't in the book. There is only one brief mention of an upcoming war in the book, and Howl makes a lot of money providing services to the military. The anti-war message in the movie is entirely Miyazaki's. So is Sophie's hard-working patience and Howl's gentle, kind spirit. The characters in the book are much more grey morally, with Sophie being angry and bitter, and Howl being a selfish womanizer. You know the 'little mouse' sexual harassment scene in the movie? In the book, it's Howl that harasses her, but he backs off when she doesn't like it.
What does Miyazaki take from the book? This is the most fascinating part of all, to me. It says a lot about Miyazaki, who was a manga artist originally. He took images. Suliman sitting in her chair bolt-upright in an opulent room surrounded by serving boys? That's a vivid visual scene in the book, but the woman isn't Suliman and her relationship with Howl is totally different. The appearance is copied almost identically. In the book there's a wizard fight with a big swirling black cloud that is cut in half by lightning, with something leaping out. That's Howl fighting the Witch of the Waste. No airships or military wizards are involved. Calcifer blazing up into a huge multicolored mass? That's in the book. The melting blob servants? Those show up in the book very briefly in one scene. The book is full of gorgeous imagery, and the movie only resembles the book at all because Miyazaki took those images, which usually meant keeping some shred of the book's story.
Oh, and there are no airships, no steam powered cars, no steampunk anything in the book. Those are all Miyazaki's loves that he added for his own amusement. He made them fit beautifully, but they're very much not part of the source material. Everything military in the movie is a creation of Miyazaki, with no presence in the book.
So, yeah, I enjoyed studying how the book was adapted, since it came out so different. What about the book itself?
I liked it. The tone reminded me of that S. Morgenstern classic, the Princess Bride. People live in a fairy tale world and know the rules of a fairy tale world. It's not funny, but it is light. Sophie and Howl are interesting people with an interesting relationship. It contained only one terrible flaw to me. In the book, Howl is from our world. It's a horrible, immersion-breaking trope, and it seems completely pointless here. Still, the book is a pleasant enough read that it wasn't hard to get past.
Howl's Moving Castle: Book or movie? Both, because they're different stories, each one worth your time.