I've been reading Martin van Creveld's Rise and Decline of the State recently. I'm not quite done with it (having finished the Rise section, but not yet read the Decline section), but it has given me plenty of food for thought.
Historical assertions follow; I have not checked all of van Creveld's sources.
The overarching 'plot', if you will, is that Western European monarchies spent a lot of effort eliminating the power of the nobility, the church, and the towns, then found themselves with nobody to help them rule (for one man alone cannot rule a kingdom). So they created bureaucracies, which in turn outgrew and devoured them, creating the modern impersonal state. These bureaucracies aggregated as much power to themselves as possible and eventually, via nuclear weapons, their ability to wage total war outgrew their willingness to wage total war. Now we see a shift back towards the power of guerillas and militias who, using the technology developed by states with less bureaucratically-fixed tactics, can effectively oppose states (at least this is where I think this book and its sequel, The Transformation of War, are going). To decentralize power in order to fight a decentralized enemy is counter to the ethos of the state ("All within the state, nothing outside the state, nothing against the state."). I think there are some glorious grand strategy gaming opportunities here, which make the failure of War on Terror all the sadder, but there remains much reading to be done.
One of the interesting thrusts of RaDotS is that a lot of the social context we take for granted today came about only in the last couple of hundred years (since 1600, say) as part of a feedback loop, where state bureaucracies caused changes in order to increase their own power, which in turn allowed them to cause more changes to their advantage (van Crevelds characterizes the modern state by its permanent, impersonal, secular bureaucracy and regional monopoly on violence; much of the book is really a history of bureaucracy, which is awfully relevant for tax season).
Mapping, census, languages, and fixed centers of government were probably the most interesting elements of this transitional period from an ACKS context. Geometrically-precise mapping on the scale of national borders was not performed (at least in Western Europe) until the 15-1600s, after the development of the printing press, putting it well after the Medieval period. If we want to be reasonably accurate to the experience of ruling a domain or exploring a wilderness, then, we ought to kiss our hexmaps goodbye (at least on the player side of the table) and switch everything to pointcrawls. Likewise, William the Conqueror's Domesday Book aside, national censuses were not undertaken until about the same timeframe, with the natural interests of accurate taxation and ability to raise levies. ACKS' familial abstraction may well be too fine-grained for the period. Common national languages began to be standardized during this period, again primarily for military purposes, though as the Basques show this process is still not complete. Again, the Common tongue is a mighty powerful abstraction, and we often skip having multiple human languages. Finally, fixed centers of government are particularly entertaining from an ACKS perspective. It was not until after the printing press and attendant beginnings of the rise of bureaucracy that kings stopped travelling their own countrysides and began ruling from distant seats through proxy bureaucrats; RaDotS cites several kings of the Medieval period who travelled perpetually through their domains, checking up on their vassals and holding court as they went. This is very different from the fixed model of rulership we saw emerging while playing ACKS! This change to remote rule also saw kings stop leading their own troops (with a few exceptions like Gustavus Adolphus). Both of these changes, though, require an established, educated, loyal bureaucratic apparatus, which is something that takes a lot longer than a single king's lifetime to bring into being.
Another interesting thing to note is the relation of religion to the state. van Crevelds contends that historically, almost all rulers of domains above the size of towns have been justified religiously, with emperors typically approaching the status of living gods and always holding the status of head of their religion. An exception occurred in Western Europe, though, where Charlemagne became Holy Roman Emperor, but the pope remained head of the religion. This put monarchical interests into opposition with religious ones, rather than the harmony typical of the rest of the world, and led to the eventual loss of religious temporal power in the West, which permitted the growth of a secular bureaucracy (which, without respect for traditional religious / "natural" law, soon rendered the monarchy vestigial). "Separation of church and state" is an aberration from the 'natural order' of human history, if you will, albeit a rather durable one and one of which I am quite fond. This separation of church and state, though, begat political philosophy (which served the same purpose as religion, ie justifying the power of those who rule - I have always been dubious of mainstream political philosophy, and am now moreso) and nationalism (a secular religion if ever there were one - Nietzsche would've called it an answer to the death of god, if he hadn't been too busy fighting the German protonationalists of his day about antisemitism to get a clear look at nationalism).
So anyway, all of that has some interesting implications for the whole fighter-cleric domain dynamic.
More to follow as I continue reading.