Have you read Marie de France yet, she's my favorite. I also just finished a lecture series from the library on grail myth. It shouldn't be too hard to find. If you want source material, Chretien de troyes is quite good and kinda started the whole Lancelot thing. Also, I quite like Tennyson. He certainly has the most beautiful take on the lady of shallot. If you run across any good modern sources that recognize that a large part of current Arthurian mythos is in fact modern, please let me know. You could also try welsh mythology which is weird as all get out but a lot of the older Arthurian stuff starts there.