Which C.S. Lewis book explores four types of love, including affection and friendship?

The C.S. Lewis book that explores four types of love is The Four Loves.

Published in 1960, the work examines four Greek terms and the kinds of human relationship they represent: storge, or affection; philia, or friendship; eros, or romantic love; and agape, associated with charity or self-giving love. Lewis uses these categories to explore both the strengths and dangers of love.

The book grew out of a 1958 BBC radio series, which means it shares a broadcast origin with some of Lewis's other popular religious writing. The four terms are analytical tools rather than four completely separate emotions. In particular, Lewis emphasizes that natural loves can become distorted when they are treated as ultimate or self-sufficient.