I'm looking for a book I read. It's based on a girl who lives in a post-apocalyptic world in a tower. Two boys went out scrounging for things in the wasteland and found an apple core (for the seeds) which was taken by someone in the upper echelon. The person who took it was found dead and the boys were accused of doing it. The girl tries to help and ends up falling down a tube and somehow ends up in the game world and she tries to help the boys from there.