I have not read all of the following titles. I hope to do so soon, however.
Sir Gawain and the Green Knight
The Canterbury Tales by Geoffrey Chaucer
Everyman
Beowulf*
Piers Plowman by William Langland
The Anglo-Saxon Chronicle
Caedmon's Hymn
Le Morte d'Arthur by Thomas Malory
The Book of Margery Kempe by Margery Kempe
Pearl
You may ask why these titles I have chosen? Well, it's hard to imagine Medieval literature without Chaucer. While several of his works are significant to understanding the literary world that Medieval people imagined, The Canterbury Tales is fundamental. Then again, so are the Arthurian stories. Whether it is from Malory or the Pearl Poet, we can't separate the tales of King Arthur and his knights from most Medieval literature. Lastly, works like Everyman and Piers Plowman bear witness to the theological worldview of many people throughout Medieval times.
While the works of Virgil, Ovid, Dante, and Boccaccio were composed in languages other than English, their books are nevertheless helpful to understanding Medieval literature as their stories impacted the written works of Old and Middle English. The Bible, Dante, and Ovid are three essential sources for understanding medieval literature, according to C. S. Lewis. As someone who has read all three, I agree with him.
*Some scholars consider Old English literature distinct from Medieval literature.
