I'm considering consolidating two books into one since the setting is seeping into rules fairly heavily at this point. A few D&D setting books have done this, and I think Dark Sun would be a decent model. It follows a straight forward platform of roll stats - races - classes and so forth, just each individual part is very setting rich so by the time your done with the book you have a good idea of where the game is going to take you. I am leaning this direction at the moment since the rules companion is feeling somewhat dry.
Another option is to simply leave most of the rules out of the book and go with the setting material alone. I would simply throw in things like 'Elves receive +1 on Dexterity, -1 on Constitution', and let the reader use whatever they want as far as what rules to use. No xp blocks, no spells, no ability rules. Myself I would still stick with B/X and would likely recommend it somewhere therein. The Oculus would naturally be a separate list of rules, and something for status titles as well if I decided to use them. In a way this would work better. Instead of spelling out how a Halfling becomes a Sheriff I could simply leave it much more vague and solely in the realm of role-playing to achieve. I am not entirely happy with the changes I made anyhow so it may head this direction if I do not like where they end up. A lot of setting books went this route. Probably because there was only one set of rules in the market in the first place (AD&D probably), so it was just assumed that is what was used. I think this works well since even running 3.x I still found most of this material quite useful and easily translated for the most part. In fact I still find AD&D naturally intuitive since that is what I grew up playing. Too bad it has such a thick layer of wonkiness, to the point where you need a book of house rules to make it somewhat playable. I suppose this is why I tend to lean towards B/X mostly.
Overall I am more pleased with the non-rules material; the race lore, the Oculus, finnic history, the slant on classes, and so forth. I suppose I was intent on have the rules added in so it is essentially a 1-stop book, and you need nothing else. And there is still merit to that since at times it may not be so easy to have other rules on hand in Iraq. So, thats where I am at during the start of this week. I think I will go over some of the local geography and history this week but that will definately be in the back of my mind.