I'll vouch for Peter Brown he is an amazing scholar of the period and a very fine writer.
My father does a lot of work in the history of the Dark Ages and early high middle ages. There is a lot more going on in that time period than people give it credit for.
In particular, I've always thought it made just about a perfect template for running a DnD game.
I wish you luck with your efforts and hope to hear more about it.
Fief seems very good but is based on information primarily relevant to a far later period. As is, oddly enough, Vampire the Dark Ages. And, to my knowledge, Magical Medieval Europe.
Penguin's History of the Franks by Gregory of Tours is an excellent and easily available primary source for the early dark ages.
Be warned that medieval history in general, particularly as it is available to non-academics, is a morass of misinformation and outmoded or faddish historical paradigms. Tread carefully.
Ideas about how historical information is supposed to be interpretted change all the time for this period, particularly due to its strange nature in modern culture and politics, and often times Role-playing books aren't very aware of these issues and play into them in a most naive way.
The dark ages, however, have been generally agreed upon to be a horrible period not worthy of attention, and as a result the smaller body of dedicated works tends to be of a higher quality and the community of people who work in it is smaller and very well informed about the works of the period.
I have no information on, or opinions about, resources published for the role-playing market other than those listed above.