Einhard's Life of Charlemagne
, is central in these lectures. There are a number of 'books' affecting to provide the same translation, but usually they are out to get you to subscribe to this that or the other. Most readable translation of Einhard's short classic on Charlemage, is in my fav site, http://sourcebooks.fordham.edu/basis/ei ... n%20Family
Fordham specializes in original source material. Given all the spin Charlemagne, Constantine, et al get, it becomes important to find contemporary writers, though like Einhard they will not be objective, either.
It's REALLY hard to find good stuff on medieval history. That's why I recommend Paul Freedman's lectures on Youtube. He covers these folks much better. This is the full playlist, 22 videos in all, you should be able to just sit back and watch all 22 hours (!) here.