I've just finished reading this, The Cosmic Trilogy by C.S. Lewis. It's less well known than his other fiction works and has been described as "Narnia for adults", a quote which I dispute - Narnia IS for adults! Anyway, this science fiction trilogy was recommended to me ages ago and I should've read it ages ago. I'm not really one for science fiction which is maybe why I was hesitant but it's well worth a read.
Out of the Silent Planet, the first book, introduces the main character, Ransom, as he is kidnapped by Weston and Devine and taken to Malacandra (Mars) where they plan to offer him to the creatures they have met there as a sacrifice. It's an excellent book - the new landscapes, vegetation and various life forms on the planet are well described and you can almost see yourself there as you read. For me, it brought a realisation that we as humans assume that any life form we do manage to find out in space will obviously be less intelligent than us, which, of course, isn't necessarily the case. How arrogant are we?
Perelandra sees Ransom transported by the eldil (spiritual beings of the universe) to Perelandra (Venus), where he again explores new landscapes and meets new creatures, most notably a human-like female. This book is fascinating as it explores the question of what if Eve had been dissuaded to eat the fruit the serpent was tempting her with? What would the world be like today?
That Hideous Strength was the one I found the hardest to read. Not only the longest of the three, it's also very different and is set in rural English countryside, focussing on academic establishments. It takes a long time to get going and it's hard to stick with it but it's well worth it. I won't write any more because I'll ruin it for you!!