Princess of the Midnight Ball by Jessica Day George

Princess Rose is the eldest of twelve sisters condemned to dance each night for the wicked King Under Stone in his palace deep within the earth. It is a curse that has haunted the girls since their birth--and only death will set them free.

Then Rose meets Galen, a young soldier-turned-gardener with an eye for adventure and a resolve that matches her own, and freedom suddenly begins to seem a little less impossible. To defeat the king and his dark court, they will need one invisibility cloak, a black wool chain knit with enchanted silver needles, and that most critical ingredient of all--true love.

In a twist on the classic Brothers' Grimm fairy tale of the Twelve Dancing Princesses, Jessica Day George has created a story with a dark and mysterious evil alongside a beautiful blooming romance. 

Unlike in the original tale, Rose and her sisters--the "younger set": Orchid, Pansy, and Petunia, the "in-betweeners": Violet, Iris, Hyacinth, Daisy, Poppy, and Lilac, and the "older set" (in which Rose is included): Lily, and Jonquil--aren't just truant princesses who disappear every night because they want to.  Their mother, Queen Maude, made a bargain with the King Under Stone that he would help her to bear children, and in return she would dance for him.

Put shortly, I love this book.  Galen is fun, kind, courageous, curious, and all he wants is for the princesses, Rose in particular, to be freed from whatever evil ails them.  Rose and her sisters aren't just passive victims of their fate, and each one tackles it in a different way.  Though sometimes they feel they'll never get out of it, at the same time they never quite lose hope.  The characters are believable and lovable, or intensely hateable in the case of Under Stone and his half-blood sons.  It's well written, and impossible to put down.  I'd give it five stars every time!