I first read it, I think, when I was about 8 years old, as my mother had it for a college class she was taking. Later it just happened to be assigned reading in high school when I was around 18 or so. I continued the pattern after that, reading it at least once every 10 years or so. I'm on my 5th reading right now, maybe?
It's not part of the pulp-fantasy canon, and I can't remember it coming up with any of my other gaming buddies in conversation. It's not part of Gygax's Appendix N. Maybe it's not really applicable to D&D much at all (the tragic structure, and the sense of pre-ordained fate playing out, doesn't synch up very well with a tabletop game). Nonetheless, it's shaped a lot of my background expectations for fantasy and mythic imagery.
One of the great things about my relationship with The Once and Future King is that every time I read it, I discover new meaning -- details, references, allusions, themes -- and new depth. Each time I return, there's a new understanding that I simply wasn't ready for at a younger age.
This sort of matches the structure of the book itself, written over a period of about 20 years. The first part, "The Sword in the Stone", telling the story of King Arthur's childhood, is light and airy and frequently comical (to the extent that Disney was comfortable making an animated movie based on it); this part caught most of my attention when I was a child myself. The rest of the book, with Arthur in adulthood, was written after White had seen the effects of World War II on the U.K., and it's darker and more brooding and troubled. There's a lot to the portrayal of Lancelot in "The Ill-Made Knight" that my 18-year-old self was suddenly shocked to recognize very deeply, waiting for me like a secret map in a book that I'd already read.
Some of the wonder of White's work comes from his intimate familiarity with archaic pastimes such as falconry, hunting, and fishing. (Apparently he lived by those means in a cottage for a year or so in the mid-1930's.) Later on, he took up flying, and that experience clearly fertilized some lengthy and beautiful scenes of families of birds flying together across the North Sea. He also spent time translating a medieval bestiary (which I would recommend for some great ideas in spicing up otherwise mundane animals in a D&D game -- available online here), which itself makes an appearance in The Once and Future King, Book 2, as the Sons of Lot prepare to go unicorn-hunting.
The Once and Future King may possibly be the single best book I've ever read. Is it absolutely perfect? No, it's not.
Pre-World War II, White wrote and published "The Sword in the Stone", and I think most critics categorize the work as a comedy, even though there are some heartbreaking scenes contained within it; Arthur receives a magical education from Merlin as best he can manage. After World War II, White started writing and publishing the darker sequels, including a concluding work called "The Book of Merlin", set at the end of Arthur's reign, with one final lesson that had been, perhaps, tragically overlooked.
Now, when the whole work was put together into one book called The Once and Future King, guess what White was unable to resist doing? You guessed it: He went back and rewrote parts of the original book from 20 years earlier. He snipped out a section of a more comical adventure with Merlin's Owl, and dropped in the extended scene intended for "The Book of Merlin" in its place (leaving out the rest of that last book, but including the 4 others that came before it).
Without giving spoilers on it, this cut-and-paste scene is the single awkward blemish on an otherwise ideal piece of literature. When you're reading Part 1, this scene very abruptly interrupts other action, and it has a tone and sensibility clearly different from what's around it. White is even forced to create a Merlin-time-dilation-magic rationalization for this part, because it actually doesn't fit into the story-time allotted to it. Then when you're reading Part 4, it ends on a slightly weird note that feels something else should be coming, but it is indeed the end of the collection. And it's a little hard to see how this all-important, extra-long lesson was rather blithely forgotten by Arthur afterward, in the mid-part of the book. If you get a standalone copy of "The Sword in the Stone", apparently that still gets published in the original format.
You might see the connection with what I wrote on Monday: Things like "Artists' best stuff tends to be their early stuff" (White's most renowned and best-loved work is the lighthearted first part), and "You should read/watch stuff in order of publication" (what I'll call the long-lesson works best at the end of the overall story), and "Business corrupts art" (or whatever reason caused White to not include "The Book of Merlin" in the overall collection). Most importantly in this case: Artists going back and altering old works usually ends in tears. But at least he didn't write a prequel (well... unless you count "The Sword in the Stone" itself as being a prequel to Le Morte d'Arthur).
Oh, and one final thing: Michael Moorcock was deeply influenced by T.H. White and his most famous work, apparently having an ongoing correspondence with him for some time. According to Moorcock:
T.H. White's series had also been very enjoyable and equally tragic. White had given me some very good advice on how to write, as had Peake. White was very kind and, looking back, tolerant with me. [Interview with Zone-sf.com, 2002]