 This is one of my favorite Heinlein books. It's written for young adults. I first read it when I was in, I think, sixth grade. It takes place in the far distant future. There is a society of people who live on a giant space ark that is headed for another solar system. And over the generations they have forgotten that they are on a space ark. The story is about a young man and his friends who begin to explore the forgotten layers of the ship and begin to rediscover that there is a larger universe out there and that they are in trouble if they don't figure out how to regain control of this ship. I love this book and I recommend that everybody read it.