Out of the Silent Planet, by C.S. Lewis
A linguist (Ransom) is kidnapped and ends up on another planet. (The science is wobbly, but it's a classic travel story stretched to unexplored worlds). How will he work with humans or with those of that world?
Perelandra , by C.S. Lewis
Ransom has to go to Venus, a world where paradise hasn't fallen. He's in a battle with evil to prevent the fall from grace. Of the three books, the religious overtones are strongest in this one.
That Hideous Strength, by C.S. Lewis
Lewis explores evil that sneaks up on you. A newly married man gets involved with a social and political group that is actually ruining England. How do you extract yourself when you realize you're deep inside something wrong? (I'm reminded of both 1984 and Mrs. Frisby and the Rats of NIMH, but they were written after this one.) As often happens with Lewis, there's a combination of Christianity and mythology that comes through.
I enjoyed these, but it definitely requires accepting that they are written from a particular moral stance. The story moves forward well and makes a fairly quick read.