The Book of Merlyn by TH White was originally a fifth book of The Once & Future King. On the whole, I think the publishers were right to 'exclude' it, though some of the best parts were inserted into the earlier parts of the book. It is a bit too polemical as an antiwar manifesto, as perhaps could only be the case when the pacifist White was facing WWII.
Pray for Thomas Mallory, Knight, and his humble disciple, who now voluntarily lays aside his books to fight for his kind.
The story, such as it is, is Arthur on the eve of The Battle of Camlann with Mordred, is reunited with Merlyn and some of his other animal tutors, who rag mercilessly on the human race for being horrible warlike monsters. The episodes of Arthur with the ants and the geese are here, and they fit somewhat better philosophically in this argument about warfare and nature, but are much better put into the Sword in the Stone, which I guess White did in later editions when it was clear 'Merlyn' was not going to be published at all.
The version I have is apparently the first publication of it in this form, published by the University of Texas from the TH White papers kept there. The introductory essays are also illuminating, particularly about White's sadistic streak that brings to mind Agravaine. Also, for an academic press, the illustrations by Trevor Stubley are surprising and excellent.
On the Map, by Simon Garfield
An enjoyable romp through mapmaking from the very beginnings to Google Earth and Skyrim. Lots of entertaining details.