Whenever I approach the topic of people who were writing history in medieval Europe the opinion of their work tends to be pretty negative. Arthurian legends are usually taken to describe why the entire process of writing history was full of inaccuracies and fictions. I would be really interested to find out if there were any notable exceptions to this, who possibly took a more analytical approach to understanding the past?
Bede is pretty widely regarded as a fairly reliable medieval source, as is Gregory of Tours. Both, of course, have their moments of fancy and conjecture, but if you're looking for a starting point, those are two decent well regarded medieval sources