A good place to start is Wickham, Framing the Early Middle Ages: Europe and the Mediterranean, 400–800 (Oxford: Oxford University Press, 2005). This work is much more forceful in emphasizing the DIScontinuity between empire and medieval kingdom, rather than the continuities (which tend to be the focus of most modern scholarship). For a catastrophe narrative for the end of the empire, see Heather, The Fall of the Roman Empire: A New History of Rome and the Barbarians (New York: Oxford University Press, 2005). For a narrative that relies much less on catastrophe as instigator, see Halsall, Barbarian Migrations and the Roman West, 376–568 (Cambridge: Cambridge University Press, 2007).
Wickham more or less uses fiscal structures, aristocratic wealth, estate management, settlement pattern, peasant collective autonomy, urbanism, and exchange networks to frame his arguments. He identifies areas where post-Roman states based on taxation (mostly in the East) were more capable of "fixing" their problems than those based largely on aristocratic land ownership (the West). He further subdivides regions in the west where aristocratic landownership continued to be the norm, and therefore to maintain some semblance of Roman structure, after the collapse of the Empire, especially Gaul and Italia; this contrasts with areas where land ownership structures either never existed or were quickly lost in favor of tribal structures (namely, areas north of the Rhine and beyond the Channel; the Balkans; rural Greece; and sections of pre-conquest Spain). This line of argument runs quickly into the Roman villas as discreet socio-economic units of organization, and how they variously translated into the early medieval social units of emerging feudalism. Wickham does not spend as much time on this aspect as others, however, and constantly reminds us that the transitions were complicated, regional, and resist blanket theories and explanations.