Inside Out\Outside In by Maria V. Snyder

This is a Harlequin Teen paperback, but looked intriguing.  The premise is interesting - the setting is a completely enclosed cube of four levels, self-sustaining, with most people, 'scrubs', working at boring jobs to maintain 'Inside', while the 'uppers' served as managers and had more space for living quarters.  The main character, Trella, a teen loner, is Queen of the Pipes, or super at her job of following a robot vacuum as it cleans the inside of air ducts.  She's spunky, but has few friends.  However, she quickly gets involved in helping an outcast foment a small rebellion.  Trella ends up as reluctant leader of the rebellion. Of course Inside turns out to be space ship.  This is actually two books combined into one - Inside Out/Outside In - to explain more would give away the plot. The plot constructs and quirks are more interesting than the characters; many of them are rather wooden.  The story is rather slow in places. 608 pages.