Scholars and amateurs alike argue whether a historical Arthur existed in 5th or 6th century Britain. The "chivalric" Arthur is an invention of the 12th century. Here is how you can access some of the literature created by the medieval fascination with Arthur and his court.
Geoffrey of Monmouth wrote a History of the Kings of Britain in the 12th century which helped establish Arthur in the medieval historical imagination. (Arthurian passages from the Camelot Project)Charlemagne
Chretien de Troyes, a 12th century French poet wrote the first influential romances of Arthur and his knights, and gave prominence to the story of Lancelot and Guinevere. His tales of Cliges, Erec et Enide, The High History of the Holy Graal, Lancelot, or the Knight of the Cart, and Yvain, or the Knight with the Lion are all available at OMACL.
Sir Gawain and the Green Knight is a famous fourteenth-century northern English Arthurian story.
Sir Thomas Mallory, a fifteenth-century Englishman, wrote the last and most influential (for modern English readers) Morte d'Arthur (University of Virginia e-text).
Charlemagne, the great emperor of the Franks, is most definitely a historical figure, but his knights, like Arthur's, accumulated many fictional exploits.
The most famous of Charlemagne's knights is the subject of the early epic, The Song of Roland (OMACL).
Like Charlemagne, a real person (a great warrior of 11th c. Spain) reinterpreted by later writers.
The Lay of the Cid (OMACL)
General Literary Sites
There is far more on the Web and off than is listed here. A good place to start is the Online Medieval & Classical Library. For Arthuriana in particular, see the University of Rochester's Camelot Project.
Historical Materials on Knighthood and Chivalry Index