What was the title of C.S. Lewis’s first book published under his initials “C.S. Lewis”?

The Pilgrim’s Regress was the first book C.S. Lewis published under the initials “C.S. Lewis.”

Published in 1933, The Pilgrim’s Regress marked Lewis’s public emergence as a Christian writer after his conversion. Earlier in his career, he had published poetry under the name Clive Hamilton. His first poetry collection, Spirits in Bondage, appeared in 1919, and Dymer followed in 1926 under that earlier pseudonym.

The new initials signaled a change in literary identity as Lewis turned toward explicitly Christian themes. The book’s subtitle, An Allegorical Apology for Christianity, Reason and Romanticism, identifies its purpose and intellectual range.

This question is easy to confuse with The Problem of Pain, published in 1940, or The Allegory of Love, published in 1936. Those books helped establish Lewis as a major Christian thinker and literary scholar, but The Pilgrim’s Regress came first under the name by which he became famous.