Historia Regum Britanniae is a highly mythologized, and possibly fictional, history of Britain prior to the Anglo-Saxon conquest. Published in 1136, the work covers 2,000 years between the Fall of Troy to the reign of Cadwallader the last non-Saxon king. Geoffrey claims to have translated an ancient...
I feel I need not speak at any length about the dubious amount of actual history contained in this influential little volume, nor would I bet money on Monmouth believing in the strict historicity of his work when he wrote it. Monmouth does, unfortunately, believe a little to readily in the patriotic...
Don't read it in one go. Full of legends and names. Apart from that, awesome in it's own right.