AAC will be run on Dreamlyrics. It will use standard 2nd edition Nobilis rules, and most of the "canonical" setting. Many details will be filled in as the campaign advances. I do not have a complete "worldbook" written yet. I have not yet populated the world of AAC with all the important NPCs that will appear later. (I have decided on a few, and some will be described below, but others will appear when they are needed.) The PCs will have some influence on the shape the world takes.
Keep in mind that Nobilis is a game where any PC can radically alter the world as we know it, permanently and even retroactively. Reality is more fluid and less linear than humans would like to believe. History has been altered more than once, and entire continents have been brought into being and then made to vanish. Entire concepts have been deleted, others have been changed. (Rabbits used to be aquatic mammals.) Even when a major miracle rewrites creation wholesale, inconsistencies can appear in the cracks, and older Nobles will remember the way things used to be. So don't be surprised when a country you've never heard of suddenly becomes a focal point of geopolitics, or the GM tells you "Why yes, many universities do have a Diabolical Studies department." Perhaps your character is puzzled when he notices people wearing flir clothing, not remembering any color flir, but experienced Nobles become quite blase about such things -- unless it affects their Estate.
Note that these changes are not wrought often or casually, first of all because if it's something the world's human inhabitants notice (like the sudden appearance of a second moon, or fish gaining the ability to talk), then widescale Dementia Animus is likely, and that means Lord Entropy slapping down the offending Noble hard. Secondly, even if it is a subtle change, or one made retroactive so that all the world's mundane inhabitants believe that there have always been two moons, or that fish have always been able to talk, other Nobles will notice it, and inevitably someone's Estate is going to be affected in a manner that displeases them. (The Power of Gravity may not appreciate the extra work a second moon will give him, and the Power of Language may have some delicate plan in the evolution of speech upset by fish suddenly factoring into his Estate.)
As the Nobilis rulebook describes, the AAC setting is much like our world, but with more wonder, more magic, more faith, and more evil. It is in many respects "darker" than the world we know, but it is not the World of Darkness. Corruption runs deep throughout society, thanks in large part to the efforts of the Cammora, but there are also Nobles who work to inspire wonder and joy. Horrible things lurk at the edges of reality, and monsters are real, but so are angels and heroes. Most people shut their eyes to the Mythic World and any hint of magic, but a few seek it out.
Not everything in the world is a Noble or Execrucian conspiracy. Not every miraculous event or remarkable individual is involved in the Valde Bellum. However, very little is untouched by the War. The world most people believe in doesn't really exist -- beneath the surface, the Nobilis, the Execrucians, and the spirits of creation are constantly doing battle. Humanity is in a bottle bobbing on the surface of the ocean, unaware of the waves constantly altering its course and threatening to submerge it.
My greatest inspirations are Neil Gaiman's Sandman graphic novels, the Dark is Rising series by Susan Cooper, and In Nomine.
The major countries -- America, Great Britain, France, Japan, etc. -- are the same. But in AAC, the Soviet Union never fell. It is Entropy's catspaw, and the Cammora finds it far too useful to allow it to collapse as it did in our world. Thus, NATO is still waging a cold war with the USSR, and the nuclear threat has never diminished. The Gulf War of the 90s and 9/11 didn't happen in AAC.
Some technology is more advanced. It's rare but not impossible to encounter "comic book" technology -- laser weapons, battle suits, teleportation machines, etc. These usually have been provided by the Cammora to their favored agents, or more rarely, were "leaked" from someone's Chancel. Lord Entropy becomes most displeased if advanced technology becomes too widespread or noticeable. Most people are not astounded when they see technology that's not widely available -- "everyone knows the government has secret technology."
Other technology has not advanced as quickly; the Internet exists, but remains the province primarily of the government and academia. Personal computers are pre-Pentium, and cell phones are expensive toys for the rich. The advance of consumer electronics has been greatly inhibited compared to our world.
Cults and secret organizations are common. There are fewer atheists.
Democracy is weak, and usually a charade. Almost every country leans towards fascism. "Innocent until proven guilty" is an alien concept. The United States Constitution guarantees rights only for property owners. (Everyone else gets "Common Rights," which still leave Americans better off than most of the world, but they have very little protection from police powers.)
The gap between the rich and the poor is larger. The police serve the rich, and maintain law and order in poor neighborhoods only inasmuch as is necessary to prevent riots. (There are exceptions some cops honestly want to help. Many come from poor backgrounds themselves. But most are apathetic at best, brutal at worst.)
Mythic Earth is a realm of pure animism, where everything has a spirit because everything is alive. Pebbles, raindrops, and rays of sunshine have spirits. Companies have corporate spirits, schools are inhabited by education spirits, and even stray thoughts and dreams are spirits in their own right. Keep in mind that most such spirits are as dumb as the prosaic objects they represent you can coerce the spirits of footprints and windowsills and beer cans to talk, but they don't have much to say.
Chancels dot the landscape, with entries to and from Prosaic Earth. Some are small, no larger than a house. Others are worlds within a world.
Nobles refer to Yggdrasil, the World Ash, but not everyone believes that creation is literally a tree, with its roots in Hell and its boughs in Heaven. Some say Yggdrasil is literally the spirit of creation. Others say that even the void between worlds is inhabited by spirits, but spirits too abstract for Earthly minds, and so what is vacuum and occasional hydrogen atoms in Prosaic reality becomes a tree or some other metaphor in Mythic reality.
When Powers venture away from Earth, it is possible to perceive the pathways to other worlds as a tree. But some claim that each world is a bubble floating in the primordial sea of creation, and they travel between worlds on cork ships. Others perceive creation as a mountain Hell in the lava-filled, subterranean depths, Heaven at the summit, and the other worlds resting in valleys at various points along the slopes. Others have more abstract representations.
There are philosophies and secret societies (called "cleaves") even among the Nobilis. Most of them are tolerated Lord Entropy and most Imperators don't really care what a Noble believes or where his true alliegances are, so long as he fights the Execrucians and obeys the Code Fidelitatis.
Aaron's Serpents exist, but I am not fond of them as Imperators. The only known Aaron's Serpent on Earth is Jormungandr. Most remain on the World Ash (or in the World Ocean, or the World Void...).
Be aware that with the exception of Lord Entropy and his Powers, none of the NPCs described in the rulebook necessarily exist in AAC. I might bring some of them into the game, when it suits me, but most do not exist. In particular, I favor classical angels and demons, and am more likely to use the likes of Michael, Gabriel, Azazel, and Semyaza than those named in Nobilis. Likewise, True Gods and chancels will frequently draw on classical mythology.
I have added the factions of Order and Chaos to my setting (though neither figures prominently on Earth).