A few thoughts on what makes the book special:
Out of the Silent Planet was published in 1938, over twenty years before Yuri Gagarin was the first man in space. Lewis is a medieval literature scholar, but he does a good job of imagining space travel, a view of Earth and the Sun from outer space, and the effects of being outside Earth's gravity and on a low gravity planet. Some of it is outlandish, like in chapter 4 (my page 30), where Weston says of being in the spaceship, "You will soon get used to it...The ship is roughly spherical, and now we are outside the gravitational field of the Earth 'down' means - and feels - towards the centre of our own little metal world." It seems to imply that gravity is produced by the largest spherical shape in the vicinity, not the most massive object. Anyhow, it's still impressive if you consider that no one had been in space or seen a picture of Earth from space. This can be illustrated by a collection of cover art for the book in its nearly 80 years of being in print.
Next, Lewis uses the space setting to speculate on the nature of matter, energy and existence. It's a dip into the world of Einstein by a man who lives in the world of Dante and Milton. In chapter 15 (my page 94) a sorn is explaining the nature of the eldil, angel-like creatures which live everywhere in the universe but are only discernible by some. He says, "Body is movement...If movement is faster, then that which moves is more nearly in two places at once...You see that if you made it faster and faster, in the end the moving thing would be in all places at once, Small One." And then he connects that idea to the nature of light. "The swiftest thing that touches our sense is light. We do not truly see light, we only see slower things lit by it, so that for us light is on the edge - the last thing we know before things become too swift for us. But the body of an eldil is a movement swift as light; you may say its body is made of light."
Lewis is a supernaturalist. He doesn't believe the natural world, what we are able to see and touch, is sufficient to explain morality and reason and what it means to be human. But his view of "supernature" is that it is a higher nature. It is just as real, even more real, than what we see. It obeys laws, just different and higher laws. Science fiction proves to be an effective medium for Lewis to communicate these ideas.
The climax of the book is Weston's speech before Oyarsa and Ransom's tortured translation into the martian language. Weston has killed some Malacandrians and Oyarsa, the ruler of the planet, demands an explanation. Weston's skill in the local language is limited so he asks Ransom the philologist to translate. Weston's speech is quintessentially modern and secular. From chapter 20 (my page 135) he says "Life is greater than any system of morality; her claims are absolute. It is not by tribal taboos and copy-book maxims that she has pursued her relentless march from the amoeba to man and from man to civilization." Here is Darwin and Neitzsche and Marx.
And Ransom must translate. "He says that living creatures are stronger than the question whether an act is bent or good - no, that cannot be right - he says it is better to be alive and bent than to be dead - no - he says, he says - I cannot say what he says, Oyarsa, in your language. But he goes on to say that the only good thing is that there should be very many creatures alive."
In a lifetime of writing books and essays, Lewis critiqued secular modern thought as irrational, self-contradictory and insufficient. But on the fictional world of Malacandra he makes the same argument using Hemingway's "show, don't tell" technique.
The denouement of the book happens after Weston's trial. Oyarsa has heard enough from Weston and now wants time alone with Ransom to ask his own questions. He wants to know about Thulcandra, the silent planet, which is shrouded in darkness to the rest of the rational universe. We aren't privy to their conversation, but in chapter 21 (my page 141) Oyarsa concludes with, "You have shown me more wonders than are known in the whole of heaven." Here is Lewis's most explicit insertion of Christian theology. This is a near repetition of 1 Peter 1:12 where the Bible's teachings about Jesus's death for sinful men are "things into which angels long to look." Oyarsa is one of those angels, longing to look.
If Lewis would have inserted some explicit Christian message here it would have been jarring to the story line. Likely those who don't know the Bible will miss it. But the idea is that something very special has happened here on Earth, a planet full of sin and misery. There is divine grace in the universe that can only be seen in contrast to that sin and misery. Lewis takes us away from our silent planet to enable us to look back and understand it a little better.
