Mary Stewart's Merlin Trilogy consists of The Crystal Cave, The Hollow Hills and The Last Enchantment (plus The Wicked Day, which is part of the same continuity but from a different viewpoint). The series, as you might guess from the collective title, tells the Arthurian legend from Merlin's (and later Mordred's) point of view, from Merlin's early childhood to the Battle of Camlann. It takes a Demythtification approach (except for Merlin's visions and ability to light fires without a flint, which appear to be real).