Guy Halsall - Worlds of Arthur - Facts and Fictions of the Dark Ages.
The title is a little misleading as it is really his attempt to make sense of the sources and archaeology to work out what the Anglo-Saxons were up to before and then after the Romans quit Britain. Proper academic, not someone on a fantasy trip. I think he rates Gildas as the most useful written source. Be aware that there's a lot of poor amateur writing in this field.
As an alternative delve into Peter Heather, who Halsall has issues with. Heather loops back to a recasting of older ideas and sees the barbarian peoples as being more cohesive. Heather takes more of a Europe-wide view so you need to find the A-S bits in his work. The big 2005 and 2009 books are where to look I think, but I would start with Halsall.
Then if you want to go up a level you need to check out Walter Goffart who died earlier this year.
The A-S don't seem to have behaved as similarly as most other migrating groups of the era did. The field is a contested one with opposed 'schools' and of course not far away are all sorts of race theories.
Without giving too much away, Halsall not only says that 'they were already here' when the Romans quit as they had been invited for defence etc. (which is an orthodox view, and that there's then significant top up migration), but thinks WHERE they were in England is not quite what many assume.