Stella recommends Book of a Thousand Days by Shannon Hale

One thousand days stuck in a tower for something that you didn't even do seems like an incredible injustice. Dashti manages to make the best of it, not so much for her own sake, but for the sake of her mistress - the reason she's in the tower in the first place. Based on a Grimm's Fairy Tale, Shannon Hale again dazzles with a deeply rich story of love and loss and redemption.
Publisher description:
Fifteen-year-old Dashti, sworn to obey her sixteen-year-old mistress, the Lady Saren, shares Saren's years of punishment locked in a tower, then brings her safely to the lands of her true love, where both must hide who they are as they work as kitchen maids.