There is a really good podcast called "the british history podcast" that goes into great detail on the history of britian starting from prehistory , and pretty much the entire anglo-saxon history is covered as they just hit the William the conqueror era. Which marks the end of the anglo saxon era.

Otherwise just reading up on the kings and kingdoms of that era on wikipedia will also give you a pretty decent picture. And you can always check the sources there.

England didn't really exist back then, there were a few different kingdoms making up what is now England, chief among them were Wessex, Mercia, East Anglia, Northumbria and Kent.

So when the vikings came, they didn't come upon a united "england" as it were, but a bunch of powerhungry warlords/kings fighting amongst themselves, if they weren't fighting the british/welsh. And even sometimes within the kingdoms itself, civil war or power struggles within their own ranks wasn't exactly uncommon either.