If you're looking to get started two books that I would recommend are Britain After Rome by Robin Fleming and World of Arthur by Guy Halsall.

The first is an archaeological examination of post-Roman Britian up until about year 1100. It deals with the Anglo-Saxon "migration", the Viking Age, and the consolidation of the Medieval Kingdom of England. However it focuses heavily on material culture, that is physical objects such as art pieces, weapons, buildings, and physical remains of men and beasts that were buried. It also follows legal, religious, cultural, and social developments on the island over the course of several centuries.

The second book delves more deeply into the immediate aftermath of post-Roman life and ends before the Vikings show up. It is specifically focused on dispelling pop history books that purport to explain the "real" King Arthur, but along the way you'll learn alot about post-Roman Britons, Saxons, and others. This book focuses more on the political and military history of the Island of Britain following the collapse of Roman authority.