‘The Historia regum Britanniae’- a famous middle English literary period’s book was written by-

Historia regum Britanniae, (Latin: “History of the Kings of Britain”) fictional history of Britain written by Geoffrey of Monmouth sometime between 1135 and 1139. The Historia regum Britanniae was one of the most popular books of the Middle Ages. Source: https://www.britannica.com/topic/Historia-regum-Britanniae