Book of a Thousand Days - Shannon Hale

When Lady Saren refuses to marry a man she fears, she and her maid, Dashti, are locked in a tower with just a tiny flap open to the outside world. As food runs low and the weather changes from broiling hot to unbearably cold, it is all Dashti can do to make them comfortable in their dark prison.

Not long after their confinement begins, Saren's suitors arrive-one welcome, the other less so-and she orders Dashti to speak to them. Impersonating Lady Saren is a crime punishable by death, but Dashti will have to play the role many times if she is to save them both from the tower and the dangers outside. As she takes control of their desperate situation, Dashti begins to understand her own astonishing talents and believe that even a low-born maid can find true love.