According to what I've read about the Once and Future King books, this volume was initially intended to be included with the other four books in the omnibus printing. It was rejected due to wartime paper shortages, and was finally published a few years after White's death. I wanted to read this because I had liked The Once and Future King so much, and I was disappointed to find that this book was less a conclusion to that Arthurian tale, and more a philosophical look at war and its causes and effects.
The Once and Future King ends unresolved, though White sets it up so that we're pretty sure that the downward trend of the story will continue. He ends that story definitively here, but he does so really with the first chapter and the last two chapters. Arthur is despondent on the night before his final battle with Mordred, but he is then reunited with Merlyn, who takes Arthur off to the cave where he was imprisoned by Nimue. There, the two of them meet with the animals Arthur met through his transformations to discuss the nature of war. This convinces him of the futility of war, and when he returns to the battlefield, he convinces Mordred to make peace. Unfortunately, even that victory turns sour when a knight draws his sword to kill a snake, which is seen by the other side as treachery, and the battle erupts anyway.
The conclusion of the legend is appropriate and tragic, but it doesn't really tell us much more than what was revealed in The Once and Future King, and the rest of the book is more philosophical in nature. In addition, Merlyn has Arthur undergo two additional transformations, but when the book was initially rejected by the publisher for inclusion into the book, White went back and added the two scenes to The Sword in the Stone, making those chapters redundant here. In fact, I wonder if the book was initially rejected for its content and not for the paper shortages, since much of the book seems superfluous at best. Much of the discussions of war could have been included in an essay or a non-fiction book.
Those who enjoyed The Once and Future King might want to read this for its curiosity value, but I don't see it as an essential part of the Arthurian tale. It just doesn't add enough to the story to make it worth reading, unless you're curious to see what White thought about war before having to participate in it himself.