I can't remember the name of this book?

It had a similar storyline to beauty and the beast, with the 'prince' turning into a great polar bear every night/day. The heroine had to make her way into an area surmised of troll-like creatures, in which she becomes a servant of the court. To prevent her prince marrying the head 'troll' she makes a mask to look like one of the trolls, as well as wearing one of three dresses she made whilst staying with the prince. I'm sorry this is convoluted but its nearly one in the morning here, and I cannot for the life of me remember the name of the book. Thanks