The Historia Regum Britanniae was written by Geoffrey of Monmouth circa 1136.  In it, he provides a mythological history of the British Isles, from the first human settlement by descendants of Troy to the end of the Arthurian Age.  It contains many stories from Britain's mythology, including the oldest known version of King Lear and one of the earliest chronicles of the reign of King Arthur.  Many details are different from later, better known versions of these stories.  Despite not reflecting accurate history, this work remains a fascinating seminal piece of British literature.