Centers of Power is a way not only to conduct the famed implied "end game" of A/D&D, but also a way for game masters to game the internal and external political machinations taking place in their own campaigns, and a way to ease player characters into those machinations so that it's not an abrupt "Oh, you're a 9th level fighter now-- you can establish a freehold in the wilderness and start collecting taxes" in a complete vacuum.
You could also play it as a complete game in and of itself, playing the power struggles within a kingdom or three among the various factions within them.
Essentially, CoP takes the various factions that make up any land; clergy, aristocracy, merchants, etc. and treats them as "characters" in the A/D&D sense. Each has attributes; power, wealth, nimbleness, etc. that determine how well the faction functions in any given endeavor, on a 3-18 scale. A small-town thieves' guild might have a power of 4, while the army of a kingdom might have a power of 14. The Golden Horde of Genghis Khan would have a power of 18. That same small-town thieves' guild might have a nimbleness of 15, because there are few people in the decision-making pipeline, while the army might have a nimbleness of 7, because once it starts rolling it's hard to stop. These attributes would change as the fortunes and makeup of the faction wax and wane.
Each type of faction (analogous to character classes) determines what it can and cannot do, analogous to character class on the individual level. An aristocracy faction might be able, for instance, to have "marriage" as an ability, which would increase its influence with another aristocracy faction, and possibly with other factions as well. A merchant faction would be able to start new trade routes as an ability, which would increase its wealth, but which would possibly be interdicted by other merchant factions seeking the same trade route.
In addition there are rules for individual characters to be their own factions (the above-mentioned 9th level fighter) and for player characters influencing the course of factions of which they are members. So a high-level thief would be able to possibly nudge his thieves' guild underworld faction in a certain direction and contribute to its own power and other abilities as he rises in level and accumulates magic items.
The supplement as a whole is not focused on the military, economic, or political, although individual faction types will necessarily be focused on one or another, and will be able to aid or interfere with other factions with their own abilities. Like a noble house using its influence at court to prevent an army from exercising certain strategies, or an underworld "family" blackmailing the head of a clergy faction into taking a certain course.
Faction actions are taken on a larger scale than individual actions, of course; where a player character operates on the scale of segments, rounds and turns, a faction would operate on days, weeks, and months. A faction having control of certain magic items, or more commonly geographic features, has an impact. If a military faction controls a fortress, its power increases when its engaged there. If an underworld faction controls a crystal ball, its awareness increases. You get the idea.
I've played out some very basic stuff on my own, and it looks like it will be a terrific system. Unlike other attempts at doing things on this scale, it "feels" right because it is essentially using the same systems used by individual characters, just on a different scale. Abilities on a 3-18 scale, type abilities like class abilities, etc.
It will not be part of Adventures Dark and Deep; this is something that would be applicable to any A/D&D-like system anywhere (although it will certainly be compatible with Adventures Dark and Deep). All you'd need to do as a game master is stat out the various factions in your campaign (and you can use dice to figure out the faction ability scores; just like it can yield interesting results to figure out why a cleric has a low wisdom and high dexterity, so too might it give fodder for imagination if a military faction has a low power and high awareness).
So I've not stopped work on the Adventures Dark and Deep Bestiary, but I am taking a short amount of time to work on Centers of Power concurrently. I hope you'll be pleased with the result. I'll keep you posted here!