I really liked this book but I don't understand the whole half-royal and mistress thing. She might not be completely royal, but he should have asked her if she wanted to become his mistress. I also had an issue with the obsession with her being pure. I understand that this book is set in a fantasy world, with morals much like the Regency period but if he'd asked (or, hey, not listened to her bitch of a sister to start with) so many things could have been avoided.
That said, I liked the world. It reminded me of Anne McCaffrey's Dragonriders of Pern, but is different enough to not seem like a knockoff.