The Historia Brittonum, or History of the Britons, traditionally ascribed to Nennius, was probably written in the early ninth century. Its account of events in early Britain provided much of material for later Arthurian legend.
Free online texts
Gutenberg: History of the Britons, multiple formats.
Internet Archive: Six old English chronicles. Ethelwerd's Chronicle. Asser's Life of Alfred. Geoffrey of Monmouth's British history. Gildas. Nennius. And Richard of Cirencester, translated by J. A. Giles. Multiple formats.
Yale Law School Avalon Project: History Of The Britons, translated by J. A. Giles.
Internet Archive: Nennius vindicatus. Über entstehung, geschichte und quellen der Historia Brittonum, by Heinrich Zimmer (1893). German language commentary.
Librivox: History of the Britons - public domain audiobook.
The Great Conversation: Further reading at Tom's Learning Notes
Gildas: On the Ruin and Conquest of Britain.