History books would be a good start.
I don't have much knowledge of the area, Mus?, sorry I can't be of more help. I'd be interested in learning a bit more about it, though my next research time zone is Europe post Roman Empire, as there is quite a bit of false hearsay that masquerades as historical fact about the time, and I would like to use that as a running start to launch myself into medieval European (i.e. non-British) history.