One of the most important developments of Heinlein's is that he brings to his generation starship story a strong religious theme. His story takes place hundreds of years into a starship's journey, and the inhabitants have long since forgotten what the purpose of the ship was. Since there are no windows on the ship, they have come to believe that the ship is their entire universe, and they have no comprehension of an "outside", yet alone that the ship is actually moving through the vastness of space. A complex religion has developed on the ship which posits "Jordan" as God, the Creator of the Ship. "Scientists" are the ship's priests, and they preach from the ship's technical manuals, which they interpret metaphorically. The ship's voyage has become the "Trip" that everyone takes upon death before arriving at the "heavenly home" of "Far Centaurus" (the ship's original destination). The work required to keep the ship operational has become thoroughly ritualised, with a reenactment of "manning landing stations" becoming a "religious ceremony".
"Universe" opens with the protagonist, Hugh Hoyland, exploring the more dangerous regions of the ship in which he lives. These regions, being closer the the centre of the cylindrical, rotating starship, are almost weightless (due to lower centrifugal forces). They are also the home of the mutants, who attack (and eat) other members of the crew. Hoyland's world is turned upside down when he is abducted by a short mutant, Bobo, and brought to one of the mutant's leaders, the two-headed Joe-Jim. Joe-Jim educates Hoyland on the true nature of the ship, taking him to the Main Control Room where he shows him the stars.
Hoyland discovers, of course, that Jordan was the name of the foundation which created the ship, and that the ship is moving through space on a journey to Centaurus. Returning to his own people to enlighten them about the true nature of the ship, and the falseness of their religion, Hoyland is accused of heresy and imprisoned, facing imminent execution. Joe-Jim, however, breaks him out of jail and takes him to safety, while at the same time abducting one of the ship's chief scientists. "Universe" ends with this scientist, Ertz, being shown the stars himself, and coming to believe, as Hoyland does, that they should attempt to steer the ship to its destination.
"Common Sense" was, in my opinion, a rather weak follow up to "Universe". The interesting themes of the ship's inhabitants mythologising and ritualising the ship and its history in order to create a complex religion is not really developed, although it certainly does still remain, primarily in the opposition between the conservative scientists (religious fundamentalists) and the modern, free-thinking scientists who accept the truth about the ship. "Common Sense" tracks the attempts of Hoyland and his companions to liberate the non-mutants from religion, while attempting to stop a bloody war between the "normal" humans and the mutants and steer the ship to a nearby star system.
What I found almost unbearable about this story was not the reductive and simplistic presentation of religion as the ignorant and backwards mythologisation of science—I was expecting that when I picked up the book—it was the absolutely terrible treatment of women in the book. According to D. A. Houdek at The Heinlein Society (surely not an unbiased source) and M. G. Lord writing for the New York Times (slightly more credible), Heinlein often created strong female characters and "terrific women", although the latter source does admit that he has "been attacked for being misogynist." After reading "Common Sense", however, I am certainly inclined to tend towards the Heinlein-as-misogynist viewpoint. There are no strong female characters in "Universe" or "Common Sense", and I'm not sure there is a female character that isn't savagely beaten by one of our "heroes" at some point in the story. During the story, Hoyland takes two wives, and attractive young wife (purely for sex) and an older wife (for domestic duties). Towards the end of the story, when Hoyland must feed matter into a "converter" in order to power his landing shuttle, he is tempted to chop up and feed in one of his wives, though his friend convinces him to feed in some of his precious books instead. I admit that I haven't read anything else by Heinlein, although I certainly intend to (Stranger in a Strange Land is on my shelf, glaring at me), and the gender politics in his later work may be much better. That said, I was far from impressed with this aspect of "Universe" and "Common Sense", which pretty much ruined the stories for me.
"Universe" and "Common Sense" were collected as Orphans of the Sky in 1963, even though they only provided enough substance for a short novel due to large font sizes and wide margins. Nevertheless, the the book has been very popular and remains in print after almost 70 years (available via BookDepository or Amazon.com).
Oh, also, the woman shown on the right of the book's most recent cover (above right) is only in one scene of the book. She is the mutant's knife maker, and Joe-Jim asks her to make him several long knives (swords). When she refuses, Joe-Jim beats her into submission — problem solved! Grr... So irritating! And I have absolutely no idea why the Astounding cover shown above has "Universe"'s two main characters walking around in their underwear. Very disturbing.