The book opens as King Arthur prepares himself for his final battle. Merlyn reappears to complete Arthur's education and discover the cause of wars. As he did in The Sword in the Stone, Merlyn again demonstrates ethics and politics to Arthur by transforming him into various animals.
White was inspired to write this book, upon his determining that the key theme of Malory's Le Morte D'Arthur is to find an "antidote for war". Rather than containing a distinct plot, this book reads more like a discourse on war and human nature.
Originally submitted for publication in 1941, due to wartime paper shortages, White was unable to convince his publisher to include The Book of Merlyn as part of the collected edition of The Once and Future King (which was first published in its entirety in 1958). Perhaps this is also due to this book's philosophical and plot-light nature. He nevertheless managed to include parts of this rejected text; in the course of revising The Sword in the Stone for the collected edition, he incorporated suitably adapted versions of scenes from The Book of Merlyn. This has the unfortunate consequence, now that both The Once and Future King and The Book of Merlyn are available, that parts of The Book of Merlyn appear to be rehashing things White has already covered earlier in The Once and Future King.