I thought Peter Heather's "The Fall of the Roman Empire", published a few years ago, was excellent, but his "Empires and Barbarians: The Fal of Rome and the Birth of Europe" is even better. In this new volume, Heather shifts his focus to concentrate on the barbarians (he simply uses the term to designate peoples outside the Empire) and also extends his time frame through the Year 1000 by which time powerful states were emerging across northern and western Europe. Heather rejects the old simplistic picture of hordes of barbarians crossing into the Roman Empire to pillage and plunder, but he nonetheless defends the reality of "mass migrations" playing a major role in what happened (although the label of "mass migration" might sometimes be more because of the impact of the event rather than because of sheer numbers of barbarians involved). Heather provides a supremely intelligent look at a very complex subject, and he carefully lays out his arguements with detail, requiring the reader pay careful attention. At the same time, however, Heather does employ a witty style to engage the reader's interest and to make his points.
If I might be so bold as to summarize what I see to be the author's central theme: Heather believes that the wealth of the Roman Empire quite naturally flowed into the lands beyond the Empire's border (through trade, if nothing else), that increase in wealth inevitably resulting in social inequalities and complexities in those neighboring cultures. This new wealth permitted some individuals to assemble small bands of elite warriors that permitted those individuals to amass even more wealth and gain additional stature (and possibly raid within the Empire to seize even more wealth). Over time, these warrior bands grew and combined in pursuit of greater ambitions towards more wealth, until the Empire itself was overwhelmed. These coalitions in turn provided a basis for the rise of powerful post-Roman states. A similar, but later process led to Slavic dominance in Central and Eastern Europe. Of course, Heather's anaysis is far more complex and subtle that this brief summary and deserves to be closely studied.