Even after the Normans conquered England, Old English (the oldest form of the vernacular) continued to be spoken throughout the country. It continued to be used in books produced in monasteries there for at least a century after William the Conqueror’s invasion.
One excellent example of this is found in the Old English Illustrated Herbal. Originally made in Canterbury in the early 11th century, this manuscript contains Old English translations of a collection of Latin remedies, illustrated with numerous paintings of plants and animals. blogs.bl.uk