Kings and Queens Of Early Britain (1990)
by Geoffrey Ashe (1923-)
This book is about the people who ruled in the island of Britain from earliest known times to 899, the death of Alfred, the only British ruler to be called “Great”.
It begins with the mythological names provided by Geoffrey of Monmouth, dismissing most of them and explaining a few. It ends with Britain divided among the Anglo-Saxons, Welsh, Danes, and Scots-Picts. In the middle is the uniquely British matter of the once and future king, Arthur.
Ashe has dealt with Arthur in a separate book, and I don’t think there is anything about him in this book that isn’t in The Discovery of King Arthur. His theory of Riothamus, the King of the Britons, acting against Saxons in Gaul, and disappearing from history after defeat, seems fairly convincing to me, and could be the basis for an interesting story.