Vikings would be another great sourcebook. Again you have the historical, the legendary and the fantasy versions.
The Aztecs are interesting but much more alien, harder to run as well I would suspect as it seems a much more structured society whereas the normal style of most players fits the Celts or even better the Vikings like a glove – and for the Vikings at least even a little dungeon bashing was not unheard off as they could and did loot old tombs so to use an example I do know of famous, named swords were recovered and returned to use after time in a barrow.
Aztecs in particular or Mesoamericans in general?
Horse nomads – Scythians are pretty interesting...
An African book would be very nice, I was bitterly disappointed that Nyambe was so fantasy heavy.
Even South Pacific islanders, though maybe not enough for a book of its own. An anthology of historical cultures perhaps:
Stone Age, Polynesians, Inuit, Native Americans, wild west...
Basically enough to do a few scenarios or short campaigns with. Most of us could probably do one ourselves, probably the Vikings, at short notice but the others would require more work to do it justice, taking time better spent gaming.
As I type this I am also trying to watch 'Sense and Sensibility', the rerun of last Sundays episode that I slept thorough, and have always been tempted by the Regency setting. Not quite so much the drawing room dramas though I owe it to the long gone and much lamented Lace and Steel that pointed out that in some settings how you dress was as important as your sword – and it used the same system for verbal duelling as for fencing which was a real eye opener. It is hard to grasp that this is the same setting as Hornblower (and his many siblings) and Sharpe.
At the very least we have some ideas for S & P article submissions!