I am running a campaign in Unthalass, which has been going on for close to a year now. It is a more or less modified and customized Unther/Mulhorand though, not FR 3E canon.
IMC, Mulhorand is not a "good" country - I altered any good gods to neutral gods - and I used the 2E Warriors & Priests and Wizards & Rogues portayal for some of their customs - arcane magic is considered a heresy in Mulhorand, only allowed for members of the clergy of Toth. The whole war played out like in the FRCS, but again, Mulhorand is not that good for Unther. They have a strict theocracy, sort of a communist economy with every factor controlled by a temple or church, and the churches and temples are full of ambitious, ruthless clerics out for their god and themselves, not neccessarily in that order. If you are not a member of the clergy you are nothing (but they still think that it is better to be a slave in Mulhorand than to be free elsewhere - not that all slaves agree, of course).
Unther had been under the thumb of Gilgamesh, but with a more liberal economy. Again, things were only good if you were a member of the upper class. The remnamts of free Unther are holed up in Messemprar, and under the influence of the Red Wizards of Thay. with rebels roaming the country.
Both Unther and Mulhorand are, IMC, egypt-flavored and almost the same culture apart from the political system and the stance toward arcane magic.
The party was originally going to free Unther and put one of their own on the god-king's throne (one PC was a descendant of an Untheric god of long past), but they ended up getting seduced by the dark side, so to speak, when they began working working for the military governor in Unthalass to gather information before starting their crusade. After the player of the would-be king switched PCs the whole party seems now to be firmly in the camp of the Mulhorand forces, with one fighter starting a career as a cleric of Mulhorand and a new PC a cleric of another god of Mulhorand.
As far as information and maps are concerned, the Old Empire spourcebook provides some, but not in that much greater detail than the FRCS map, as far as I remember, but my memory may not be that reliable, since I hardly use detailed maps, I get by by using the overview maps for travel, and making up the rest.
Scott Bernie has an updated site for 3E Old Empires, but I have the adress not here right now.
I suggest using a lot of historical information about old Egypt for flavor.
I made a small private campaign sourcebooks with NPCs and places, and background, but it is in German, so I doubt you want to use it.