hello aspirant

The event that began the transition from Old English to Middle English was the Norman Conquest of 1066, when William the Conqueror (Duke of Normandy and, later, William I of England) invaded the island of Britain from his home base in northern France, and settled in his new acquisition along with his nobles and court. William crushed the opposition with a brutal hand and deprived the Anglo-Saxon earls of their property, distributing it to Normans (and some English) who supported him.

you can also read David Daichess' A Critical Histroy of English Literature.

you can also look for essays in JSTOR, which is an online library for students.

Andrew Sander's book in History of English Literature or cambridge Guide to History of Middle English Literature can also help you.

hope this helps.