The first section of The Once & Future King, in which Merlin teaches the young Wart (short for Arthur) is definitely YA. The later parts are definitely more adult. I was in high school the first time I read it. That same year I read Rosemary Sutcliffe’s Sword At Sunset (a “realistic” version of the story of that leader who wasn’t really named Arthur & who was probably not a King, either), the C S Lewis Space Trilogy & LOTR (the edition with the psychedelic covers).
Re-read* The Once & Future King* fairly recently & it held up.
I’ve caught a few episodes of Merlin & will probably Netflix it someday. Arthur’s story (& Robin’s) can survive some really weird retellings. Besides–Giles plays Uther!
Hold on, what Wikipedia cites is the chapter in Higham’s King Arthur on the modern historiography of the debate on his historicity which argues that the issue has indeed been much disputed. It even opens with the sentence, ‘The historicity of Arthur has been deeply contested across the twentieth century’ (p. 11). So that’s a perfectly appropriate citation to support the statement that, ‘his historical existence is debated and disputed by modern historians.’ More to the point, the Wikipedia entry includes a whole sub-section on ‘Debated historicity’.
I’ve had Thomas Berger’s Arthur Rex, which some Doper (alas, I apologize, I don’t remember who) recommends strongly, on my shelf since my last bout of Arthuriamania, when I read the Mary Stewart series a few years ago (it’s amazing, BTW, definitely read it if you haven’t), so that’s probably next. I also realize it’s been 20 years since I read Mists of Avalon, so I’m thinking of rereading that.
Re: the TV show – I really am enjoying it tremendously. Excellent casting for the four main characters (Merlin, Arthur, Guinivere, Morgana), plus the very toothsome Anthony Head as Uther. I’m about 3/4 of the way through Season 4 and hope to finish the whole series in the next week or so.
BTW, while we’re talking all things Merlin – I’m also immensely fond of the 1996 minseries with Sam Neill as Merlin and Helena Bonham Carter as Morgana.