Supposedly 500-1500 AD was a dark age which produced history's greatest art, books, some of its most munificent literature, polyphony, classical music and such trifles as saving civilization from Islam; development pluralistic democracy, constitutions, and the idea of natural rights; along with advanced science and math.
The list of great medieval literature runs into the many hundreds and rather uniquely, includes many women [see Diane Watt 'Medieval Women's writing']. Medieval works covered the entire spectrum of taste from mythical Arthurian romance and myths; to astronomy and how to build a church. The practical to the mystical was covered by different genres and styles. The printing press – 1451 – built on centuries of innovations and nascent market demand; released a flood of literature ranging from the small pamphlet to massive tomes that needed 2 men to portage.
Even the Encyclopedia Brittanica admits that Medieval literature was profound and rich. Drama, the arts, musicals, stories of all varieties were printed in their thousands and before 1451, painfully rendered by hand, creating masterpieces of art-work [see the Kells for eg]. Importantly, the spread of literature and literacy, was prompted, but not dominated in total by the Church. Secular themes are all rife.
“One of the best known morality plays was translated from Dutch to be known in English as Everyman. A large majority of medieval literature was anonymous and not easily dated. Some of the greatest figures—Dante, Chaucer, Petrarch, and Boccaccio—came late in the period, and their work convincingly demonstrates the transitional nature of the best of medieval literature, for, in being master commentators of the medieval scene, they simultaneously announced the great themes and forms of Renaissance literature.” [ibid]
One of the best medieval sagas is the 6th century Swedish tale of Beowulf, a commentary on society, honour, valour and dragons with accurate descriptions of dinosaurs which only adds to the delight of reading this book [how could someone describe a T-Rex without seeing one, or a pterosaur] ? The 'Beowulf Project' is an online endeavour by the University of Kentucky which allows the general reader access to the translated medieval manuscript, line by line. Many such resources now exist for medieval literature. The interest in medieval manuscripts, many which are supreme works of art as well of the intellect, is growing.
Some notables from the medieval period would include:
-Song of Roland: between 1140 and 1170
-Villehardouin, Conquest of Constantinople: ca. 1207-12
-Joinville, Vie de Saint Louis: ca. 1309
-Gregory of Tours, History of the Franks: completed in 594, albeit the surviving manuscripts date from later periods (the oldest fragment from the 7th century, others from the 11th and 15th centuries)
-Nibelungenlied: ca. 1204 (or a little earlier)
-Saga of the Volsungs: the text known under that title is an early 13th century transcript of a much older legend, originally transmitted orally
-Ditie de Jehanne d'Arc, by Christine de Pizan, 1400
-Ecclesiatical History of the English People, by the Venerable Bede ca 731
-Scivias, by Hildegard of Bingen ca 1150 [more information provided on this important nun, who was a spiritual and secular leader is here]
A female writer one Julian of Norwich, circa 1380 wrote a remarkable work on the Trinity and human love named, 'Revelations of Divine Love', a very complicated and interesting metaphysics focusing on the mystery of the Trinity. Very little is known about the woman behind the book.
A list which allows you to vote for the best Medieval literature [most of the books listed are from the medieval era, a few are not]; is here at goodreads.com