Which Tolkien work was originally delivered as the 1939 Andrew Lang Lecture?

On Fairy-Stories was originally delivered as the 1939 Andrew Lang Lecture.

Tolkien gave the Andrew Lang Lecture at the University of St Andrews on 8 March 1939. The lecture examined the nature and purpose of fairy stories, including their relationship to fantasy, childhood, recovery, escape, and consolation.

A revised version was later published in essay form. It became one of Tolkien’s most influential statements about imaginative literature and is frequently read alongside his fiction. The essay explains why he regarded fairy stories as serious works rather than merely children’s entertainment.

The title can be confused with Tolkien’s famous 1936 lecture “Beowulf: The Monsters and the Critics,” which was delivered at the British Academy. That earlier lecture concerned the Old English poem Beowulf. “On Fairy-Stories” instead developed Tolkien’s ideas about the literary value of fairy-story traditions and the creative possibilities of what he called the sub-creation of imagined worlds.