The story circles around Finn, who may or may not be the long lost crown prince; and Claudia, the daughter of the Warden of Incarceron, engaged to be married to the remaining prince. They live in a world not too different from another's. Finn is stuck in the true prison of Incarceron and Claudia, in the gilded cage of the Outside. The Outside is frozen in an earlier century, ironically through the use of technology (called Protocol) and advancement in anything is forbidden.
There are a number of interesting twists that if it were a movie, might come across like a cross between Matrix and Labyrinth. Incarceron is a very cold and mechanical place, contrasted by the organic facade of the Outside. Definitely worth a read. It's pretty well written and has lively enough characters. Admittedly, their roles aren't that original, but Fisher doesn't let that get in the way of the story.
The next book in the series is Sapphique. If I get around to it (my to-be-read-list is rather massive... >.>), Incarceron was good enough that I would give book two a try.
My library is directing the book towards the teen audience, though I would be willing to suggest it for early tweens (Amazon suggests 12+). The text is clean and if I remember correctly, there was no graphic violence.
-main characters could've had a more original twist, but it doesn't hinder the story.