I was bought this book as a present, from someone knowing I like fantasy, and my first impression was that it was intended for younger readers. But I read it anyway and was pleased I did. It may indeed be for younger readers - I do not know - but the characters are likeable, the setting is interesting and the plot is just complex enough.
Yet I think what I liked best was simply the atmosphere: at the end of a hard day it is the perfect book for escaping with. Recommended.
Note: this review is quite definitely not of the movie (reviewed elsewhere). I will just say that, if you have only seen the movie, the book is very, very different.
I see there is a semi-followup called Castle in the Air. I have not read it but people say it will appeal to those who liked Howl's Moving Castle, so I may get it soon.
Amazon UK: Howl's Moving Castle and Castle In The Air
Amazon JP: Howl's Moving Castle and Castle In The Air