Book Of A Thousand Days by Shannon Hale

A young scullery maid vows to protect the sheltered noblewoman she serves after they are betrayed and locked away in a remote tower; over the course of a thousand days she records their struggle for survival, the cruelty and kindness they encounter, and the fears and discoveries that shape her. Facing isolation, hunger, violence, and political deception, she uses resourcefulness and fierce loyalty to shield her charge, uncovers hidden truths about identity and honor, and grows from powerless servant into a courageous, compassionate agent of change and love.