Incarceron by Catherine Fisher

Finn is a prisoner. In the world of Incarceron everyone is a prisoner, no one enters and no one escapes. But Finn knows he's from the Outside, he has painful seizures where memories of another world, a world outside, overtake him. When a crystal key finds its way to Finn he knows this could be his only way out.

Claudia is the daughter of the Warden of Incarceron. Trapped in her own kind of prison, Claudia suspects a treacherous plot involving her father and the Queen that took the life of her betrothed, Prince Giles. Determined to find out what happened, Claudia breaks into her father's study and finds a crystal key, a key that could answer all her questions.

Complex and brilliantly written, Incarceron is a fast paced adventure from the depths of a malevolent prison to the outside Realm, a world with its own dark side. I'm already declaring Incarceron one of my top 10 of 2010! Watch for the sequel, Sapphique, coming January 2011.