Incarceron

I don't want to give too many details, as there are secrets and twists and turns, but the two main characters, Finn and Claudia, are well done and grow more interesting as the story progressively connects them in ways they couldn't even dream of. The other characters are all well-drawn, and the idea of a self-contained, sentient "being" as a prison - in fact, the whole concept of a prison that is a world of its own - is different, also well done. But the theme of any prison story is, of course, escape. I wonder sometimes if, in the world Fisher describes, escape is possible - even "Outside" is a min-numbing prison of sorts, set up by a ruler who feared change.
One thing that bothered me: her descriptions are sometimes too vague. I occasionally had a hard time picturing a setting or place she was describing because her descriptions just didn't contain enough details, or details that I could translate into a visual image in my head.
Good book, and I would recommend it if you like YA fantasy. I am looking forward to the sequel - but don't read any blurbs about it if you are still reading Incarceron! I made that mistake and saw a spoiler I wish I hadn't.