Another one down on my "Reread Heinlein" project. Four left to go!
It was originally published in Astounding, in two parts, five months apart, in 1941. It's a masterpiece of plopping the reader into a bizarre tech/social setting, and only eventually revealing "what's really going on".
But I'll tell you, stop reading if you object:
A slower-than-light starship, designed to travel to a distant star system over a couple generations, has gone horribly wrong. A mutiny has killed most of the crew, leaving the worst in charge. Over the years, the survivors breed, some of them mutated. The cylindrical ship still rotates, providing "gravity" to the inhabitants. "Lower" high-gravity levels are occupied by the non-mutated. The "higher" levels hold the "muties". Conflict is common, and cannibalism is practiced. The origin and purpose of the ship gets lost in mythology. The world is the ship.
Into this comes Hugh, a wannabe "scientist". He's captured by the muties, and one of their clan, a two-headed "twin" named Joe-Jim, takes him to the Captain's Veranda, where he can see the stars. Gasp! And so starts a plan to fulfill the ship's mission. But there's a lot of bloodshed along the way. (This may be Heinlein's most violent book.)
According to the Wikipedia page, Heinlein revealed the ship's ultimate fate in Time Enough for Love. So I'm looking forward to that.