For a Real World/Alternate Earth RPG Supplement you really need it to be shallow.
Seriously.
Look at Glorantha. Gloranthan Supplements have a lot of background, in-depth history and so on because Glorantha is a made-up world and doesn't have the background material.
Real World/Alternate Earth isn't like that. I can go to the library and pick up as many books as I want about almost any ancient culture. They have histories, timelines, maps, descriptions of the political factions and so on. There is more information about the Ancient World on my bookshelves at home than there is about the whole of Glorantha and almost none of it is from RPG supplements.
So, give us a potted history of the area. Give us a short decription of the political factions, basic maps, descriptions of major cities and cultures. We cna look up the rest. The Internet has so much to offer in this regard that any Real World/Alternate Earth supplement should link to a web page with many more links for background and extra information.
For a RPG supplement we really need hard stats for things. For RQ that means stats for Races, Backgrounds, Professions, Equipment, Weapons, Religions and Magic.
If you have to wade through hundreds of pages of background material then you end up skimming through it and don't appreciate the depth of the material.
RQ3 Vikings and Land of Ninja were a bit skimpy on the background but were even more skimpy on the religions/cults. Mythic Russia (for HeroQuest) has an awful lot of background, arguably too much, but didn't skimp on religions and cultural stats, but that is a big book and I can't see Mongoose producing books of that size. Stupor Mundi had a bit of background, although it could have done with a little bit more, no maps but had reasonably good wirteups for the Religions and Cultures.
So, I am all for shallow when looking at RW/AE supplements.
After all, if you want an area written up in more depth you can always bring out more supplements. And that can only be a good thing, surely.