I haven't read Malory's version yet (yet, I say). Chretien de Troye compiled the Arthurian stories into a romance before Malory. Geoffrey of Monmouth is one of the first Arthurian writers, with his History of the Kings of Britain. Of course, he tried to say Arthur was absolutely real, prophecies, magic, and all. (most of the other kings in his account were real, after all). Brutus wrote his accounts a little later, and also talks about Arthur. Both seem to be trying to tie England to the Roman Empire in an age when it was considered a backwater.
You can always check out the Celtic, Gaulish, and German myths the Arthurian stories are based on. If you like the Arthur/Guinevere/Launcelot dynamic, the Celtic "Tristan and Iseult" (famously adapted by Wagner, whose version is referenced several times in Eliot's Waste Land).
Malory was the first to write in English, of course. If you're looking for good modern versions, read Steinbeck's King Arthur and His Knights of the Round Table. Stewart's Crystal Cave is good, though it follows Merlin (I think it was one of the first, if not the first, book to re-tell the Arthurian stories with another character as the protagonist). There's always Tennyson's fabulous Idylls of the King.
Eliot's Waste Land is actually an Arthurian quest story, set in/around/behind (whatever your opinion is) a post-modern mind. It's primarily referencing the Fisher King that Percival/Parzival meets.