A classic of the science fiction genre, a defining one for the trope of generation ship.
Hugh Hoyland lives on the Ship. After a venture to low weight decks, where he's endangered by a mutie attack, we find the humans living in farms on the high weight decks, and he hears the religious text on the Mutiny and is inducted into the ranks of the priestly scientists.
It's when he ends up a prisoner among the muties that he learns much of what the astute reader has pieced together earlier. The tale involves a two-headed mutant, lots of books, a violent war, the question of whether what Hugh sees is real, the Converter in duplicate, and more.
fun but very spoilery fact: [Spoiler (click to open)]In a later Heinlein work, characters mention that the ship was traced. They found a surviving human colony on the moon, and a completely dead ship.