I am a HUGE fan of Hayao Miyazaki's animated film adaptation of Howl's Moving Castle so when I saw that the entire Howl's Castle Book series was on sale on my Nook I just had to snag it. I will say that just like any original source material for a beloved movie you should go into the book knowing that it will be different from the movie.
Right off the bat the description for Sophie and her back story gives you a lot of information that you were missing previously in regards to who Sophie is as an individual and her personality. I also enjoyed getting to know a bit more about her family, especially her siblings.
Once Howl entered the picture I fell in love with him just like I did in the movie. His personality shines a lot more in the book and his motivations and emotions are a bit clearer. I also enjoyed reading the dynamic between Calcifer, Howl, Sophie, and Michael and learning how this group grew to care about one another.
I also enjoyed the story of the Witch of the Waste a lot more than I thought I would. She was a soul who lost her way due to fear, jealousy, and temptation. The sub plot in this revolving around Howl's true origin and learning about his family was wonderfully refreshing.
If you haven't read the book I highly suggest it!