A little context about the question-- I'm writing a book where the characters are loosely based on characters from Arthurian legend and, well, frankly my idealistic side shot ahead of me. I know very little about Arthurian legend and I've been trying to supplement my ignorance by reading wiki articles and encyclopedia entries. I've since come to the realization that I've underestimated the density of the subject and overestimated my ability to learn it rapidly and keep schedule. I'm likely to throw this project on the back burner until I feel comfortable enough with the subject. So, I'm looking for something old and thick. Ahem, to read, I mean. I know very little about King Arthur. I know the legend and the key figures but not in great detail. I know it has fishing lines in many different moats (plz don't crucify me). It has its place in history, mythology, and literature but I don't really have any idea where one ends and the other begins. What can I read to make me an expert --or at least sound like I have any idea what I'm talking about?
The first thing to remember about Arthurian legend is there isn't just one cycle of myths; it is a series of stories, ballads, poems, folk tales, romances, and legends spread from Wales to France. The Arthurian legends were primarily local and passed on through oral tradition until Geoffrey of Monmouth wrote his Historia Regum Britanniae in the 12th century; you can read the Arthurian excerpts (translated into English) here. From there, Arthur and various other characters tied to him became popular material both in the British Isles and in France, especially with Chrétien de Troyes, who added the character of Lancelot and the Holy Grail as important elements.
The closest thing to a "definitive" source to begin would be L'Morte d'Arthur (1485) by Sir Thomas Mallory - this is a compilation, translation, and synthesis of material from 13th century French Arthurian romances (the Vulgate Cycle and Post-Vulgate Cycle) and Middle English sources like the Alliterative Morte Arthure and Stanzaic Morte Arthure. Mallory is the ultimate source for the popular image of King Arthur as epitomized by Alfred, Lord Tennyson in The Idylls of the King (1859-1885) and T. H. White in The Once and Future King (1958), both of which were based on L'Morte d'Arthur.
The Welsh stories contained in sources like the Mabinogion (written down in the 14th-15th centuries) are also worth reading, if only because they are so different from the French romances - rather than dealing with Christian relics and issues of courtly love and fidelity, the Welsh have Arthur fighting Saxons and giants.
That doesn't really begin to cover the whole of Arthurian legend and literature - but they are the go-to sources for most later depictions of King Arthur - these are the primary sources whose reprinting in the 19th century helped spur a new craze for Arthurian fiction.