Goodreads:
Once upon a winter's night, a poor crofter trades his daughter Camille to wed Prince Alain of the Summerwood in exchange for a lifetime of riches. Though love blossoms between Camille and the prince, he is haunted by sadness and will not allow her to see his unmasked face. Believing she can lift whatever curse has been bestowed upon him, Camille acts on her own-- with devastating results, all she loves is swept away.

Now, to regain what she has lost, she must embark on a desperate quest through the hinterlands of Faery, seeking a mysterious place lying somewhere east of the sun and west of the moon....