You probably already know about the Sword in the Stone. But don't be fooled. The rest of T.H. White's Once and Future King is decidedly not a children's book. It's classic mid-century internationlist liberalism, also an exploration of how private foibles can affect public life, and a textured discussion of the motives of the personalities involved in one of our great mythic tragedies.
The post-Sword business is split up into three books which focus on 1) "Queen of Air and Darkness," about the Orkneys and Arthur's original sin that produces Mordred, 2) "The Ill-Made Knight," about Lancelot, particularly Lancelot and Guinever, and 3) "Candle in the Wind," about a childish, publicity-seeking princess. Well, actually "Candle in the Wind" is about the climactic, tragic, confrontation with Mordred.
What makes White's telling so compelling is the way he invests the characters with motivation and humanity, and because he weaves his tale over the course of decades as one coherent story, not simply a collection of events that takes 25 years to happen. Lancelot's not merely the greatest knight, he's the greatest knight because he's spent his life training to be merely worthy of upholding an impossibly high standard. And when we see Guinevere and Lancelot together, just before their downfall, it's as a couple entering middle age, whom we've seen spend a lifetime deepening their love from youthful passion.
The political story - and this is White's real purpose - is likewise developed over decades. The younger generation of knights is restless because they've never known any king other than Arthur. And Arthur's increasingly desperate and sophisticated efforts to channel evil into something constructive follow, loosely, the path of western civilization as a whole. White is desperate himself to find an answer, but his answer - with the hindsight of over 60 years - is ultimately unsatisfying.
White was writing during some of the darkest days of WWII, and was thinking not only about the horrors of war, but also about its fundamental causes, and how to avert future ones. But he was also a typical mid-century liberal. Mordred is, in The Once and Future King, a stand-in for Hitler. As such, he must be fought. By the time White published the Book of Merlyn in 1958, Mordred was Stalin, and need be accomodated. And the ultimate answer, naturally, is to subsume national ambition to the UN. Of the three approaches, only the first - confrontation with evil - proved to be effective.
White never realized that the answer to his problem had already been discovered. Sure, we needed to establish the rule of law, but Arthur did that and found his kingdom unable to survive its logical inconsistencies. While Arthur was able to tolerate the deception of his wife and friend, the law couldn't tolerate their subversion. The problem was the fusion of the personal and the state. Depersonalize the state, and those particular inconsistencies disappear.
And so we're left with a strange disconnect: we care deeply about the characters and mourn for their losses, and we mourn for Arthur's failure as king. But we don't really mourn all that much for the idea, because by the end of the book, we're not so sure we understand or like the idea at all.