Ever wonder what the material the TGS staff looks up for the Arthurian legend? Here is the list, compiled by Todd Jensen.
Written in the 12th century (around 1136, to be precise), it covers a period of British history from the original colonization of Britain by the descendants of Trojans fleeing from the sack of Troy, under the leadership of Brutus, the great-grandson of Aeneas, all the way down to the death of the last British king, Cadwallader, in exile in Rome in 689.   The focus, however, is on Arthur himself and his reign.   Geoffrey’s work is the oldest extant "start-to-finish" account of Arthur’s life, and for this reason, is one of the leading primary sources.
[It is worth pointing out, however, that there are some startling differences between Geoffrey’s account of Arthur’s reign, and the story that we know; there’s no mention of the Sword in the Stone, Camelot, the Round Table, Sir Lancelot, or the Quest for the Holy Grail, and Merlin disappears from the story after Arthur’s conception.   Most of Geoffrey’s story, in fact, is taken up with Arthur engaged in empire-building, becoming the ruler not only of Britain, but also of Ireland, Iceland, Norway, Denmark, and France - and is on the verge of conquering Rome when Mordred rebels.   The missing familiar elements were added into later versions of the story.]
The most accessible edition of Geoffrey of Monmouth is a translation by Lewis Thorpe for Penguin Books (the copy that Todd Jensen uses).
Written in the late 15th century (Sir Thomas Malory completed it in prison around 1470, shortly before his death, and it was published by William Caxton in 1485), it covers the story of Arthur from start to finish, in what has become the most familiar version of the legend.   One of the most easily accessible primary sources: found in two versions, the Caxton version (in twenty-one books, each one divided up into a number of chapters), and the Winchester version (a manuscript version pre-dating Caxton’s edition, divided into eight self-contained stories).
A collection of various medieval Welsh legends; the title refers actually to four interconnected non-Arthurian tales, but a number of Arthurian tales are included as well.   Among these are "Culhwch and Olwen" (the source of the boar Troit), "The Dream of Rhonabwy", "Geraint and Enid", "Owain", and "Peredur".   Contains much information about the Welsh side of the Arthurian legends (which is often rougher and wilder than the more cultured medieval tone of Malory).   Found in various editions (including a Penguin Books edition, translated by Jeffrey Gantz).
There are also some useful studies of the legend and the theories about the historical original for Arthur (if there was one).