What's interesting about Miyazaki's "adaptations" is how freely he discards all but the skeletal frame. Like the plot of any Road Runner cartoon, he uses the original work to setup the action, and then takes off from there. Add in his complex, roaming, episodic directoral style, and you end up with movies that are unorthodox, non-linear, and very Asian. This is not a commercial for the corporate cash cow. This is a wandering, epic journey.
We're used to movie spinoffs that are carbon copies of the books (Harry Potter comes to mind), but Miyazaki clearly doesn't have any lucrative franchises to protect. He's interested purely in telling his own stories. With Howl's Moving Castle, you get a sense of what elements resonated with him, which parts he stored away in his memories for years. Then he uses that framework to tell a greater story, one far more personal, using all his familiar icons and archetypes.
This, I'm sure, is what threw a lot of people of with Howl. They thought they were getting a movie version of the book. They weren't expecting Juliet of the Spirits instead. And you pretty much need the knowledge of his whole career, stretching back to Horus, Prince of the Sun, to understand and appreciate it. But that's what I love about Miyazaki's Howl. It's a collosus, an abstract collage of elements that should never fit, but do. Just like that walking castle.