This is a retelling of Beauty and the Beast. Vaguely. It has a daughter sacrificing herself for her family, but this time it's under duress. The father made a deal with a demon (who just happens to rule their land, somewhat harshly) so that his depressed wife would have children. The price of her having them was that one of his daughters had to marry the demon by the age of 17, and unbeknownst to the parents at the time, that the mother will die in child birth. The father has to choose between his twin daughters: Nyx, very smart and who looks just like him, or Astraia, sweet and the image of the dead mother. He chooses Nyx and trains her and treats her differently than her sister. And Nyx knows it. She's resentful and hateful. She pretends she isn't but she is. 
 She goes to the demon, The Gentle Lord, and wanders his house every day searching for a the way to defeat him, but during her time there she begins to get to know him and her attitude changes. But really, who is the beast, Nyx with her hatred or The Gentle Lord with his demon eyes and deals with a twist? 
 This is a much more adult and darker version of Beauty and the Beast than the Disney version, but since it doesn't follow the story except for the basics, that's ok. I thoroughly enjoyed the book - couldn't put it down.