C.S. Lewis’s Ransom Trilogy includes three books.
The trilogy consists of Out of the Silent Planet, Perelandra, and That Hideous Strength. The books follow Elwin Ransom, a philologist who becomes involved in conflicts involving Earth, Mars, Venus, science, politics, and spiritual powers. They were published between 1938 and 1945.
The first novel sends Ransom to Malacandra, Lewis’s name for Mars. The second takes him to Perelandra, or Venus. The third returns to Earth and shifts toward a modern social and scientific dystopian setting. The series is therefore both planetary adventure and philosophical fiction.
The sequence is commonly called the Space Trilogy, although Lewis’s own titles do not all use that phrase. It is also sometimes called the Ransom Trilogy because Ransom links the three narratives. Neither later film adaptations nor Lewis’s other speculative novels change the number of books in the trilogy.