The following summary is useful. It's from the "About this book" section.
Howl's moving castle is set in a world conceived by the late 19th century European neo-futuristic painters where magic and science co-exist. The main idea is that one day a witch of wasteland puts an 18-year-old girl named Sophie under a spell, turning her into a 90-year-old lady. The aged Sophie encounters the wizard Howl and she begins her strange life with Howl in his castle, the formidable walking fortress. The film is based on Howl's moving Castle children's book, by Diana Wynne Jones. All concept sketches are by Miyazaki. Concept art and backgrounds are by the art staff supervised by art directors Yoji Takeshige and Noboru Yoshida. Character designs are by supervising animators Akihiko Yamashita and Takeski Inamura. The background art and still images for this book were created from digital data. Scene images without captions are still images.
The book is split into 3 parts. Part 1 is about Miyazaki's concept sketches. Part 2 contains concept art, background art, character designs, movie stills and plenty of concept sketches. Part 3 is the script, or final screenplay. Throughout the book, there are breaks with narration from the art directors and animators explaining the movie's production process. They would talk about animation, colour design, research and characters. There's a short section that explains how computer graphics was used to overcome the limitations of cell drawing, which I thought was really interesting.
Miyazaki's storyboards with their rough sketches and notes on the storyline, are more informative than concept sketches and a total bliss looking at them even if you don't know Japanese. This book and hence the movie itself is a feast of colors giving emphasis on the "realness" of the characters and with soften incredibly beautiful backgrounds. It's not an exaggerations to say that this movie's art made me reevaluate my opinions about background art. These lush landscapes with their incredibly creative colour seem to be totally otherworldly beautiful.
There are several versions of the book. This one sold on amazon.com contains the final screenplay and it's hard covered.
Also check my flipping through the book: