The Princess Curse by Merrie Haskell
Reveka is apprenticed to an herbalist. She like her job well enough, but she would rather live in a convent and open her own herbery. The only problem is that she doesn't have the money. Then she learns of the curse on the twelve princesses. The king offers one of his daughters in marriage to whoever breaks the curse and if a girl breaks it, he will provide her dowry-or money for the convent, in Revka's case. So Revka sets out to break the curse.
But things are much more complicated. Anyone who has stayed in the princesses room to find where they go to dance is either found in an endless sleep or they disappear. Reveka is frightened by the terrible fates of the many who have gone before her, but she knows she must continue. With the help of unexpected friends of all kinds, she continues her quest. But will she succeed? Or will the curse get her as well?