Are there any resources about the inspirations of Miyazaki in making Howl's Moving Castle?
I don't think this book has anything about Howl's Moving Castle in particular, but Starting Point: 1979 - 1996, written by Hayao Miyazaki himself, and translated by Beth Cary and Frederik L. Schodt, is a 500 page treasure trove of interviews and other works regarding Miyazaki's thoughts and inspirations on his films. I purchased this book many years ago and only read a portion of it, so I can't really attest to how much this will help, but it's a good starting point for your research on his other films (pun may or may not be intended).

EDIT: Upon further research on Amazon, I discovered that he has another book called Turning Point, 1997 - 2008 along the same series, which does seem to contain information on Howl's Moving Castle. You might want to take a look into this.