The last chapter of the book takes place only hours before the final battle between King Arthur and his son and nephew Mordred.

White was inspired to write this book upon determining that the key theme of Malory's Le Morte d'Arthur is to find an "antidote for war".

[1] White had revised The Sword in the Stone (1938), The Queen of Air and Darkness (1939), plus The Ill-Made Knight (1940) to weave in the anti-war theme.

[citation needed] The Harry Ransom Humanities Research Center of the University of Texas at Austin purchased the bulk of White's personal papers and manuscripts between 1967 and 1969.

In 1975 the original manuscript for The Book of Merlyn was discovered amongst this collection, and was prepared for publication by the University of Texas Press in 1977, as The Book of Merlyn: The Unpublished Conclusion to The Once and Future King, with a prologue by Sylvia Townsend Warner and illustrations by Trevor Stubley.