My personal recommendation would be R. Fleming, Britain After Rome. It makes good use of new archaeological evidence to tell a compelling historical narrative, it's recent, it's credible, and it's also inexpensive and very readable.

If you're especially interested in the early part of the story (the end of Roman Britain), I would also recommend J. Gerrard, The ruin of Roman Britain. It's more expensive, so look for it via ILL. It disagrees strongly with Fleming's version of the fifth and sixth centuries, and it would be beneficial to read both and compare their arguments.

If you're interested in debates about the historical King Arthur, or just want to know more about what evidence, precisely, survives from this early period, G. Halsall, Worlds of arthur is also worth a read. He does a fantastic job explaining how historians and archaeologists have used the available evidence, the problems and limits of their arguments, and some possible new ways forward. His version of the time period is, yet again, different from the two books above, and comparing his approach to the sources with that of Fleming and Gerrard would teach you a lot about how historians and archaeologists study this time period.

I'd recommend them in that order, and Fleming is the only author I've listed who covers the entire period of Rome - 1066.