What was the name of C.S. Lewis’s first book about Christian apologetics?

The Pilgrim’s Regress was C.S. Lewis’s first book about Christian apologetics.

Published in 1933, The Pilgrim’s Regress was Lewis’s first major work after his conversion to Christianity. Its full title identifies it as an allegorical narrative about reason and romanticism, and its form deliberately echoes John Bunyan’s The Pilgrim’s Progress.

The protagonist, John, travels through a symbolic landscape while searching for the source of a troubling burden. The book incorporates Lewis’s debates with philosophical and cultural ideas he encountered in modern intellectual life. Lewis later believed that the work was too obscure for some readers and added a longer introduction to explain its symbols. It predates The Problem of Pain and Mere Christianity, which became more widely known introductions to his Christian thought.