Maybe I am being naive, but does one need a detailed primer from an established economic orthodoxy to explain this all? I always took it as a given that the chaos resulting from the collapse of a unified imperial society was sufficient on its own merits to create localized, agrarian society. A localized agrarian society is what most of western Europe had experienced before the Pax Romana - Iron Age peasants living in mud huts and hill forts under the service of some petty chief, whose main occupation was raiding the wealth of the neighboring petty chief. From that perspective, the biggest difference between pre-imperial Europe and post-imperial Europe was simply one of local religion and language being replaced by Catholicism and corrupted Latin.