Incarceron, by Catherine Fisher

Finn is a "cell-born" member of a Scum gang in Incarceron, the living prison with millions of electronic eyes and torture chambers, designed to recycled and re-use everything, even the people who are imprisoned there.  Finn remembers nothing before waking up in a cell, apparently remade from DNA of others who died, but his flashes of memory of the outside world convince him that he belongs outside. His dealings with the Scum gang lead him to a raid on a Civicry caravan, a group of generally law-abiding nature banded together for protection, and as a result of the raid he takes possession of a small device which turns out to be an electronic key which enables him to communicate with someone on the outside, a promised place of freedom.

On the outside, Claudia is the wealthy daughter of the Warden of Incarceron, living in a stagnant world in which most technology is hidden and permitted only to the Sapiente or scholar class, in restricted amounts. The populace is expected to live "in Era", replicating through methods real and false, a by-gone past of castles and peasants, ostensibly to preserve a more peaceful and pristine time, but in reality to exert control.  Claudia chafes at the restrictions and hopes to escape to Incarceron, a social experiment started hundreds of years ago in which prisoners were placed in an ideal location with all the supplies and tools they would need to thrive and the guidance of Sapiente trained to create a perfect society.  In an attempt to avoid an arranged marriage to the remaining prince of the kingdom, Claudia steals the key from her father's office in an attempt to get inside.

When Claudia and Finn make contact, they become convinced that Finn is actually Giles, the prince of the realm to whom Claudia was engaged but who was allegedly killed while out hunting.  With mounting evidence, including a royal tattoo, they begin to believe that Gile's death was staged, and he was thrown into Incarceron to make way for his incompetent half-brother and ambitious step-mother.  Beginning to realize that neither world is the haven they have been led to believe, Claudia and her teacher Jared try to find a way to get Finn out while from the inside Finn attempts to follow the path of the Sapphique, the only person ever rumored to escape the prison, along with grudging and tenuous help from Finn's oath-brother, Keiro, devotion from Attia, a girl Finn rescued from a gang lord, and encouragement from Gildas, a Sapiente obsessed with getting out. 

Alternating viewpoints within the same chapter, this story slowly unfolds to show the dysfunction both inside and outside of the prison.  Claudia's father is alternately villainous and empathetic, Keiro is shown as both a selfish anti-hero and a frightened boy trying to survive and twists and turns offer unexpected revelations.  In the end Claudia and Finn/Giles end up on the outside, the Warden is on the run and inside and Finn's friends are apparently trapped in a vengeful prison after the Warden destroys the gate behind him.  The story is ripe for a sequel with numerous loose ends, characters who are alternately heroic and villainous and twists and turns that keep you guessing.  The sequel, Sapphique, is scheduled for release in January.