Geoffrey of Monmouth was a clergyman and one of the major figures in the development of British history. Born in about 1100 in Wales, he probably had some Breton blood. After graduating from Oxford University, he became archdeacon of Llandaff and/or Monmouth, and in 1152 he rose to the position of bishop of St Asaph. He died around 1154.
Geoffrey of Monmouth wrote several works of interest. The earliest one to appear was Prophetiae Merlini (The Prophecies of Merlin), which he wrote at some point before 1135. Geoffrey presented a series of apocalyptic narratives as the work of the earlier Merlin who, until Geoffrey's book came out, was known as "Myrddin". (It is assumed that Geoffrey changed the name of the seer to avoid an unwanted association with the French word merde.) The first work about this legendary prophet in a language other than Welsh, it was widely read, and believed, much as the prophecies of Nostradamus were centuries later.
Next was Historia Regum Britanniae (History of the Kings of Britain), the work best known to modern readers. It claims to relate the history of the pre-Saxon kings of Britain, but much of it, such as the idea that Aeneas was the ancestor of the first line of British kings, is based on legend. It is one of the first texts to mention King Arthur and the first surviving text to mention King Lear (as King Leir).
Lastly, Geoffrey wrote the Vita Merlini ("The Life of Merlin") at some point between 1149 and 1151. This is Geoffrey's own retelling of the earlier Myrddin legend from Welsh tradition.
All of these books were written in Latin, as were most learned works of the medieval period.