Nennius 
History of the Britons [EPUB ebook] 

According to Wikipedia: 'Nennius was a Welsh monk of the 9th century. He has traditionally been attributed with the authorship of the Historia Brittonum, based on the prologue affixed to that work, This attribution is widely considered a secondary (10th century) tradition. Nennius was a student of Elvodugus, commonly identified with the bishop Elfodd who convinced British ecclesiastics to accept the Continental dating for Easter, and who died in 809 according to the Annales Cambriae. Nennius is believed to have lived in the area made up by present day Brecknockshire and Radnorshire counties in Powys, Wales. He lived outside the Anglo-Saxon kingdoms, isolated by mountains in a rural society.’