Howl's Moving Castle by Diana Wynne Jones

As the oldest daughter, willful, outspoken Sophie knew that her life could lead to nothing but mistakes. When her father dies, her stepmother sends her two sisters out to make their fortunes while Sophie continues to work in the family hattery. As she whispers her thoughts to the hats she trims, the shop begins to grow in popularity, until she offends the Wicked Witch of the Waste. The witch casts a spell turning Sophie into an old woman. Sophie leaves the shop and happens upon the castle of the Wizard Howl. He has a reputation for finding young women and eating their hearts but that doesn't stop Sophie from moving in and turning the lives of the whole castle, including Howl's apprentice and his fire demon, upside down in a battle against the wicked witch and for Sophie's future.