Which author co-wrote The Space Trilogy with C.S. Lewis?

C.S. Lewis wrote The Space Trilogy without a co-author.

The trilogy consists of Out of the Silent Planet, Perelandra, and That Hideous Strength, published between 1938 and 1945. All three novels were written by Lewis, although they were shaped by conversations with fellow members of the informal literary group known as the Inklings.

J.R.R. Tolkien, Charles Williams, and Owen Barfield were important colleagues or friends in Lewis’s intellectual circle, but none co-wrote the trilogy. Tolkien influenced Lewis’s literary and religious thinking, while Williams and Barfield contributed to the group’s discussions. The question can cause confusion because Lewis and Tolkien exchanged criticism and ideas about one another’s fiction, but collaboration and friendship were not the same thing.