This is the second book in The Space Trilogy by C.S. Lewis, and I do suggest you read them in order, or you might become a tad bit lost.
Our hero, Ransom, returns for yet another adventure in space. This time he ends up on Venus with a mission to do something, but he has no clue what.
He meets a green woman that is the queen of the planet and has been separated from her king, due to the unconventional geographical set up of the planet. She is very naive in the ways of life since she has been living along on her planet and the only other being is the kind. Ransom takes up talking to her, teaching her about life and things, all very philosophical.
Then an unexpected visitor shows up and ruins everything. Ransom finds out what he was sent to do and basically fights Satan to win the innocence of the green woman and her planet.
Admittedly this is not the most engaging book out there, it took me a while to read, but I thought it was better than the first book in the sense that I found the discussions between the characters to be thought provoking, and I like to think.