I know very few things about this book. Only the things my boyfriend told me. I'd really appreciate help finding it. It's an older book. Sky might be in the title ... in the future there are two groups of people one lives in the plains living off the earth. the other group lives deep in the heart of a mountain. they live in This mountain underground where they are very high tech and advanced. They Fly planes in the Mountain till they get permission to fly out side but most people go crazy when they go out side. book starts out from the perspective of a man who lives in this mountain he is also a plain flyer. The other group lives of the lands like Indians in a type of plain. Sorry I don't know much about this part seeing as it's told from a different character and he hadn't mad it that far yet.