Howl's Moving Castle
Perhaps you saw the flick by Hayao Miyazaki? Miyazaki usually writes his own stories, but he was clearly inspired by this amazing book. Miyazaki also directed My Neighbor Totoro, Spirited Away, and Princess Mononoke. Howl's Moving Castle by Diana Wynne Jones is also a must read for fans of Harry Potter! The story is about Sophie who is a hat shop apprentice until a witch turns her into an old woman and she finds herself in the castle of the wizard Howl. Diana Wynne Jones is the author of MANY books - so you will never run out of great fantasy options.