Here are some books I find helpful as we read the final chapters of Edward Gibbon's The Decline And Fall In The West, Volume 1.
- Barbarians and Romans: The Birth Struggle of Europe, A.D. 400-700
by Justine Davis Randers-Pehrson - From Roman Provinces to Medieval Kingdoms (Rewriting Histories)
by Thomas Noble - and this old standby, though a bit of a time stretch:
Mohammed and Charlemagne
by Henri Pirenne