I love reading books which attempt to seriously grapple with the question of what medieval life was actually like - mainly out of genuine interest, obviously, but also out of a desire to give my D&D a slight veneer of authenticity without being boring or pompous about it.
Here's Sumption on French kingship in the early medieval period:
[T]he reality of power was more elusive than the formulae. At the beginning of the eleventh century Robert II...exercised direct power in less than a tenth of his kingdom, a compact lozenge of land stretching from Paris in the north to Orleans in the south. Here he was the immediate feudal lord. Elsewhere he was merely king, compelled to rule through vassals who exercised the royal power for him but did so in their own names and with an independence which reduced the monarchy to a portentous honorary dignity. The princes could and quite frequently did make war upon him and upon each other, as well as maintaining direct relations with the papacy and foreign powers.
I love that: merely king. A reminder that medieval "countries" were nothing like "countries" as we understand them today. How much power does the king in your Blahblahland have?
Along similar lines:
Provinces long engulfed by the expansion of the royal domain remained nations in themselves, sustained by traditions rooted in recent history and geographical fact. Paris was infinitely remote from most of France at a time when a mounted messenger could cover only 30 miles in a day in the best conditions. Convention and conservatism isolated these communities even when geography did not. Strangers were to be ejected from parish churches, as the statutes of provincial synods never ceased to declare. Villagers marked out their territory with rows of stakes and crosses. Beyond these frontiers, there lay the cathedral city, the market town, the shrine of a local saint, little else.
"Points of light", anyone? I love the idea of PCs turning up at a village and finding it marked out by sinister-looking stakes, and then being summarily ejected as "strangers" for not having some form of legitimate and accepted customary introduction. And, on the subject of language and the medieval state:
[A] bishop of Viviers could..threaten to disinherit his nephews if they spoke French instead of the 'language to which I was born and my father before me'. Pope John XXII was born in Cahors, was educated at Orleans and reigned in Avignon, but he was nevertheless unable to understand a letter which the King had addressed to him in French.
Even better: at the village they not only don't understand "common", they virulently despise it and will cast out and disinherit anybody who even dares to let it pass their lips.