The Thirteenth Princess by Diane Zahler

In this retelling of The 12 Dancing Princesses, Diane Zahler imagines there is a thirteenth princess. The king, who longs for a son is upset when his wife gives birth to a thirteenth daughter and dies after childbirth, sends her to live with the servants. Zita doesn't find out until she is 12 that she is a princess. Meanwhile, the princesses become ill and Zita, the stableboy and Babette (a witch) set out to find out what is happening to them and save their lives. At once romantic, magical, exciting and inventive THE THIRTEENTH PRINCESS is a charming read.