Howl's Moving Castle is, overall, a good book.
That said, I don't think it is even in the same sphere of interest as J. R. R. Tolkien, or J. K. Rowling. The tone is different; DWJ maintains a witty tone, very emotional, but practical as well. She keeps you interested because of the ways her characters act. Sophie is a particularly complex character, and I enjoyed the ways in which she tried to deny her, um, feelings for Howl (especially the weed killer). I found Tolkien to be more detail-oriented, and less character-based. His books weren't as easy (or, in my opinion, interesting) to read. J. K. Rowling? A different story entirely. The three books don't compare, and not because one is better than the other. They're just completely different.
The book and the movie differ as well. At least two people I know, faced with me coaxing them into reading the book after seeing the movie, declined, with the excuse that they didn't think the book would be as good. This is definitely NOT TRUE. Miyazaki couldn't possibly summarize the character's personalities (Sophie's stubbornness and emotionalness; Howl's selfishness, and total escapism, and his courage, bravery, genius and hot! body, haha). He didn't even try. Come on. Even though I love Miyazaki, admire his films, and think he is truly skilled, I don't think this film was at all his best. The characters were saccharine. Sophie was a nice old woman without a lot of personality, and Howl was a seemingly selfish man who turned out to be a flying bird turned war hero turned 'ideal' male lead. And then there was the Witch. She WASN'T GOOD. SHE WAS EVIL. I know I sound pessimistic, and there were parts of the movie that made me love it... but as a movie. I loved this book first, and I was really disappointed with the movie. If you are looking for more Miyazaki, you're out of luck.
The book starts out with Sophie, the responsible sister of two girls constantly in competition and the daughter of a hatter and his new wife, an ex-assistant of the shop. She doesn't believe that she has much chance of striking it rich and living happily ever after. But soon, for reasons somewhat unknown, the Witch of the Waste decides to invade her shop, and turns her into an old woman! Shocked, Sophie soon begins her journey away from the hat shop to cure herself. She ends up staying as a cleaning lady for the notoriously womanizing wizard Howl. Supposedly, he eats hearts. Her encounter with him is complicated, involving two curses, unknown motivations, and all the supporting characters and sub-plots.
The best thing about the book is Sophie's personality. She is actually an honorable person. In the beginning, she seems overly dutiful, and near the middle she can seem immature; but if you look more at the reasons she acts that way, her actual character becomes clear. I like that she's human, and acts in different ways. So that was what made the book for me.
Give this book a chance! It's better than it might seem.