What was the title of C.S. Lewis's first published novel for adults?

The Pilgrim’s Regress was C.S. Lewis’s first published novel for adults. Published in 1933, it is an allegorical work subtitled An Allegorical Apology for Christianity, Reason and Romanticism.

The book follows John, a character who leaves his home after discovering a burden and encounters figures and places representing ideas, philosophies, and spiritual positions. Its title echoes John Bunyan’s The Pilgrim’s Progress, but Lewis’s book is set in a modern intellectual landscape rather than Bunyan’s explicitly named Christian allegory.

Lewis had already published poetry under the name Clive Hamilton, but The Pilgrim’s Regress was his first prose novel for adults. Later adult novels included the science-fiction trilogy beginning with Out of the Silent Planet and the mythic Till We Have Faces.