Geoffrey of Monmouth's British History.
The story, as recounted by Geoffrey, a monk of Monmouth, is, that Walter Mapes, alias Calenius, archdeacon of Oxford, who flourished in the reign of Henry I., and of whom Henry of Huntingdon and other historians as well as Geoffrey himself make honourable mention, happened while he was in Armorica to light upon a History of Britain, written in the British tongue, and carrying marks of great antiquity. He came over to England and sought after a proper person to translate this curious but hitherto unknown book; he very opportunely met with Geoffrey of Monmouth and so recommended this task to him. Accordingly, Geoffrey, being delighted with this ancient book, undertook the translating of it into Latin, which he performed with diligence
Some modern commenters have placed this book into the class of "fabulous fiction", but it deserves a place in the list of "Monkish Histories".