While it shouldn't be compared to his later genre-busting novels, Orphans of the Sky is an entertaining Robert A. Heinlein book in its own right. The concept is one often mentioned in science fiction, but rarely expanded upon: a giant, self-contained spaceship - it's own Universe - in which humans eat and breathe, sleep and breed. The twist here is that they've been in it for so long that they can't remember life before it - or even imagine it, as they now believe the the Ship IS the Universe. It is an interesting social critique, as it shows how perfectly viable truths (indeed, Common Sense) can be reduced to mere mythology and religious twaddle. The book is well-written. Short, compact - two stories in about 120 pages - it is very tight, and this is one of those rare stories where not a single word is wasted (a complete contrast to some of Heinlein's later novels, one might say.) These are also the last two stories in Heinlein's Future History (never included in The Past Through Tomorrow.) A worthwhile story, I'm glad to see it back in print. Certainly not a heavyweight novel, but Heinlein fans will enjoy it. Reccommended for them, or as a good distraction.