I just finished reading Orphans of the Sky by Robert Heinlein. This has got to be one of my favorite concepts for a sci-fi story. It's set on a giant ship that is making a voyage to a distant solar system. Several generations have passed since the launch and since there aren't any windows in most of the ship the people have stopped believing in the trip, earth and anything outside of the ship. They consider those things to be legends. When someone discovers the controls and the stars, he has a hard time convincing others that the legends are true.

When I first read this in junior high I really liked the religious undertones. It's always been one of my favorite from Heinlein, but for the longest time I couldn't remember the title, just the premise. Then I was listening to X Minus One recently and I heard a radio adaptation of the first half of the book. Then I was able to find the title, request it from the library and read it again.