This is the fifth and final volume in The Once and Future King pantalogy (after The Sword in the Stone, The Witch in the Wood, The Ill-Made Knight and The Candle in the Wind).
The day before the final confrontation with his son Mordred, Arthur follows Merlyn to the Combination Room, where lives his menagerie. There he listens to the magician and Archimedes, Badger, Urchin and so on, who are in a political debate on how the human way of considering life and the world is different from that of animals.
I was disappointed with the Book of Merlyn, which in fact is hardly a novel. Merlyn's supposedly natural history lesson is but an excuse for discoursing on war and the bellicosity of Man. The only passages where there's an actual story are when Arthur visits the ant nest and travels with the wild geese, but these chapters were already included in The Sword in the Stone. As for what happened to Lancelot and Guenever, it is briefly mentioned in the manner of history books. The introduction on T. H. White's life is interesting, and there are some nice illustrations, but as a whole I found nothing worth recommending this book.