Guy Halsall's Barbarian Migrations

Professor Guy Halsall's Barbarian Migrations is a game changer - at least if you spend time thinking about Roman and barbarian armies in the late and post-imperial period - and especially if you like to play games with those armies. 

This book is a great read for those interested in the hundred years on either side of the collapse of imperial rule in Western Europe in the late 5th Century. While it does not review the armies mentioned above exclusively, a lot of the political, cultural, and ideological engines that drove the period directly and indirectly impacted the militaristic dynamics of the times. From a gaming point of view, you might find the book challenges you to rethink what really constituted the last imperial Roman armies and their enemies. In some ways, it might be best to think of them in terms of factions instead of the black-and-white Roman v. Barbarian dichotomy. 

There's probably a number of aspects of the period's feel that have no real quantifiable impact on the gaming table. But even those are worth keeping in mind if you play historical games for the sheer enjoyment of playing a game that resonates with your love of actual history.