
I liked the book a bunch, though it did slow down a bit along the way. Part of this is a scope problem - even if sources in this area are somewhat limited, it is a daunting task to cover all of Europe and beyond over five centuries.
I do think his two main tasks are important. He does a good job of explaining how the transition of Roman power in the west took place over a long period of time; barbarians having served in the Roman army and as administrators etc. How Roman customs and forms survived with varying strength over time.
It is interesting to read how the church and the state grew in complementary ways - each needed the others. Also how land was donated to monasteries -turns out this was a good way for families to continue to control blocs of property (if not providing some afterlife insurance). Parish churches as a later development, with generally unschooled local priests becoming another control mechanism.
Tax systems - Romans maintained a land tax, which permitted feeding cities and keeping huge standing armies. Few other systems could pull this off. Kings started giving away land to nobles; which was tricky business given the finite supply.
Some strong centralized kingdoms developed, especially Charlemagne. But then devolves into local powers over time. Who become increasingly adept at limiting the freedom of peasants, skimming off more of their work time and production. Castles start appearing . . . feudalism in the offing. With weaker kings, stronger local nobility.
Pilgrimage sites as big business - wise investment to come up with a bunch of relics.
He also threw in some information about Scandinavia, Russia, Bulgaria, etc. - much later developing.