Here lies Arthur; King who was, King who will be.
--The Once and Future King
One of my favorite takes on the Arthurian legend is Gerald Morris's Squire's Tales series. Over the course of ten books, readers are introduced to a new perspective on characters ranging from Gawain and Lancelot to side characters like Enid, Sir Kai, Culwich, and Gaheris. In fact, when I took classic literature in college, I recognized several of the more obscure legends from these books. The author has also created several memorable and realistic side characters, ranging from Gawain's squire Terrance to orphaned Sarah. The last book, Legend of the King, is a tearjerker of the highest degree, because I know all the characters.
Finally, the Dark is Rising series brings the Arthurian legends full circle, when the once and future king returns to lead the ancient forces of Light to victory over the Dark. The first two books focus more on the modern day, but the Arthurian element strengthens in latter books with the introduction of Bran.