The Space Trilogy by C.S. Lewis is quite possibly my favorite work of science fiction, particularly if you define “favorite” as “books you’ve read the most”. I’ve read the entire trilogy four times, and I’ve read That Hideous Strength six times.
Book One: Out of the Silent Planet
Dr. Elwin Ransom, philologist, university professor, (Lewis based this character on his close friend J.R.R. Tolkien) is kidnapped by Dr. Weston and taken to Mars. Dr. Ransom discovers that the earth is exiled from the rest of the solar system due to its fallen nature.
Lewis presents a beautiful and thought-provoking imaginary anthropological analysis of the cultures on Mars. There is one particular scene in this book that makes me cry every time I read it.
Book Two: Perelandra
There is more of importance in this small work than in just about any contemporary work on philosophy or theology. Ransom finds himself on a type of virginal Venus. Dr. Weston is possessed by demonic forces and is attempting to destroy an Eve-like character and the wonderful world she inhabits.
Some of the best dialog I’ve ever read, Dostoyevsky-like in power and impact.
Book Three: That Hideous Strength
This one takes place on Earth, in a fictional university town in England. A think-tank like organization known as N. I. C. E. is trying to take over the world. While the leaders of N. I. C. E. talk a good game, they are opening up the world to an unspeakable evil. Dr. Ransom returns, in an albeit smaller role.
This is my favorite of the three, although it is quite different than the other two.