Main Out of the Silent Planet
This ... is about as weird as you'd expect a CS Lewis science fiction novel to be.
It's a dated book. This is a style of sci-fi that died when we realised that going to space was going to be as crude as putting a lot of fuel behind a metal lump and blowing it up. It's fantastical and descriptive and not at all focused on the technological and practical aspects.
I think perhaps for you to understand what this is really about, I need to bring a sentence forward from the end; 'it even occurred to him the distinction between history and mythology might be itself meaningless outside the earth.' This isn't a real sci-fi but more of an ideological sci-fi and as in Narnia he's suggesting that he's writing about ideas that exist if the fact doesn't. The theme of the novel is clearly religion and christianity in particular but rather than examine the idea of a completely alien religion instead he's asking how it would tie in with our world and the meaning it might have for our world.
Ransom (and it's good to note that it's given to be a pseudonym rather than some co-incidental awfully blunt symbology) who was brought to the planet of known as Malachandria by his inhabitants and spends most of the time getting to know it's culture, the concept of which is this is a civilization that has never fallen.
It's possible you might find this story blunt or offensive because there are no two sides to this. CS Lewis wrote a story with basis in how he believed the world factually was, just as religion is naturally left out by sci-fi writers who believe that it's not part of how the universe factually is. The climax of the story is a brilliantly funny deconstruction as Ransom tries to translate in small words the blusterings of a certain type of British Colonial who used Darwinism as some sort of bizarre excuse for racism and in translating it, accidentally shows it's absurdity. 'What lies in our future, past our present ken, passes imagination to conceive, it is enough for me that there is a beyond' becomes 'though he doesn't know what will happen, he wants it to happen very much' but this might too jar with those whose ideas align more with the colonial figure.
The book is strange, controversial and I wouldn't necessarily describe it as entertaining, but the ideas and descriptions are interesting
Main A Satisfying Read, if one takes a few things with a whole lump of salt
I should start off first by saying that it is pretty much impossible to review all three books with the same brush, there are too many differences between them, despite featuring the same characters and ideas.
Out of the Silent Planet: This book, written in the style of an old first person exploration narrative, is definitely the best in the series. It is a short book but Ransom's experiences discovering more about the planet Malachandra(Mars), meeting and learning about its three peoples, and learning the ordering of the cosmos is fascinating and very well handled. It is also by far the least preachy of the three books.
Perelandra: One thing that should be known to understand Perelandra is that C.S. Lewis considered it his best work. This books is also first person narration but this time it is Ransom recounting the tale to a fictional version of Lewis. Here Ransom is sent to Perelandra(Venus) to stop the temptation of Eden from happening again. While it gets a little preachy at times this is Lewis' rational/humanistic theology at some of its very best and the arguments between Ransom and the enemy are fascinating in many points and the climactic encounter is really quite riveting.
That Hideous Strength: This book is far different than the first two. It is written in typical novel form and is nearly twice the length of books 1 and 2 put together. The two biggest problems with it, however, are that you get about 1/5 of the way into it and you can predict the ending with near perfect accuracy(including how it will unfold) and Lewis is EXTREMELY preachy here, including espousing certain ideas, particularly on the role of women, that were likely outdated when he wrote the book. There is also the matter of the fact that while the core 'fight to stop evil taking over the world' plot is straightforward, the underlying idea is, by Lewis' own admission in the introduction rather confusing unless you are familiar with his essay the abolition of man. Despite all of this, the book is still very well written(it is Lewis, after all, whatever else he was, he was a good writer) and worth a read, but this one you really need to take with some serious grains of salt.
On the whole, there is more good material than bad in this trilogy, and is well worth a read for sci-fi, fantasy, and Lewis fans.