When C. S. Lewis wrote his space trilogy he called it a "modern fairy-tale for grown ups". He wasn't kidding. Sometimes pedantic philosophy aside, Out of the Silent Planet was friendly enough, a little bland perhaps, but Perelandra was plain creepy. And That Hideous Strength turned out to be one of the more macabre books I've read. I can't say, either, that I expected Lewis to have all his characters running around swearing and calling each other "bucking idiots".
In this trilogy we have everything from playful little alien cubs, bubble trees, and loveable old "Mrs. Dimble" to nudity, demon possession, and disembodied heads. Lewis proves himself to be a mishmash crossover king, incorporating 20th century earth, Roman gods, Arthurian legend, a mention of Atlantis, and Tolkien prehistory. I've probably missed something; Lewis sure didn't. (Tolkien apparently wasn't tickled to find out that Lewis had not only repeatedly mentioned Numenor without asking, but misspelled it and confused it with Valinor)
That said, I loved these books. I didn't love everything in them and don't agree with Lewis on some points - I didn't even understand him here and there - but I loved the books. The world building, theology, and artistry, not to mention story, that went into them is astounding.
A word of warning, however. Lewis takes real earth and real theology, and mixes them so seamlessly with myth that it's painful to try to separate them again. These books aren't Narnia, not by a long shot. It's not fantasy with Christian imagery. It's fantasy... but it's also Christian theology. With the two woven so tightly together, there's a danger of either assuming that some of his theology is also just fantasy or that some of his fantasy is a part of good theology. We want to swallow it whole and then ask whether it's true or not, or at least, we want to ask which parts are true and which aren't. At any rate, I did, got nervous when I couldn't nail down the distinction, and decided to stop trying before my brain imploded. In the end, I concluded that it's not a good way to approach said books.
I believe that Lewis meant the space trilogy as a story to help us reflect on the relationship between our God, his creation, and our historical human society. He means for the ideas to provoke thought and appreciation, not for the specific details to necessarily be slotted into our doctrinal statements. Unfortunately it's so easy to try to look at his work through the modern dichotomy of fact vs. fiction, to try to be rational and scientific about things (a philosophy which he tears apart in That Hideous Strength, by the way) because that's how we've been primed to think ever since we were wee kidlets being taught to answer everything with either a "yes" or a "no".
I wouldn't call the space trilogy an easy read, by any stretch, by they are very well worth it. Whatever your bent and however you take them, Lewis says a lot of really interesting things - assuming that you like philosophy. Which, if you've read to the end of this post, you probably do.
"Long since on Mars and more strongly since he came to Perelandra,
Ransom had been perceiving that the triple distinction of truth from
myth and both from fact was purely terrestrial-was part and parcel of
that unhappy distinction between soul and body which resulted from the
fall. Even on earth the sacraments existed as a permanent reminder that
the division was neither wholesome nor final. The Incarnation had been
the beginning of its disappearance."