Incarceron by Catherine Fisher

Incarceron is a vast and mysterious prison, a place from which only one man has ever escaped. Within it's borders desperate groups of prisoners fight for scarce resources. Fourteen-year-old Finn is a prisoner there who has no memory of his past. In the outside world there is a wealthy girl named Claudia whose father is the warden of Incarceron. She has been groomed her whole life to marry the heir to the throne and take over the kingdom when she is queen. However, the current prince is not her first fiánce—the real prince was killed years ago in a suspicious accident. When Finn and Claudia both acquire crystal keys that allow them to communicate with each other they band together to try to help Finn escape from Incarceron. I saw this book described as science fantasy and that's a good description. It has lots of action and the author has created a pretty amazing world for her characters to live in. Science fiction and fantasy fans will like it. (And the sequel, Sapphique, is already published.)