This is the latest Shannon Hale's young adult novel. The story is a retelling based on Brothers Grimm Maid Maleen. The novel is written in diary form by Dashti, the maid of Princess Saren. Set in Mongolia, the two are locked together in a tower for seven years by Saren's father for her refusal to marry the horrible man he desires. However, Saren is secretly engaged to another, Khan Tegus, despite her father's wishes.
And with that, I will leave you to go and get it for yourself, so I don't totally ruin it. Just know that the book is wonderfully-written, the setting is fascinating, and it has a nice dose of romance.