A small book entitled "Later Roman Britain" by Stephen Johnson", presents a clear account of this period in what was to become the very difficult times following the withdrawal of Roman rule.
For me, the most important aspect was the fact that the "Anglo-Saxon world" had made itself known to this "British Roman world" much before the "Celtic world" would tell their stories. Saxon raiding parties had caused a series of defensive forts to be built along the eastern coastline. This interaction had produced a long history of contact between these folks long before old Vortigern was accused of letting these folks get their foot in the door.
Lots of maps and lots of pictures make this read a neat book to gaze upon. First published 1982 by Paladin, Grafton Books , it has been reprinted in 1986. Later Roman Britain, a bridge to the past.