I started a campaign last thursday which will eventually involve Rashemen in a large way so I did some googling before that:
Best piece of information I found www.thieves-guild.net/index.php?pid=334
Their most common enemy, the Thaywww.thieves-guild.net/index.php?pid=336
A wiki on Rashemen containing more history info and foodsforgottenrealms.wikia.com/wiki/Rashemen
Cities and sites of Rashemen, including a map for Immulsantirwww.realmshelps.net/faerun/rashemen/citi...
Map of Mulsantirwww.gamebanshee.com/neverwinternights2/w...
You can find a 4e map of Rashemen itself in the back of the forgotten realms campaign guide, though a pre-4e map should also be available if you google for it. The good thing about Rashemen is that there are a lot of good sources available thanks to the Mask of the Betrayer NWN2 videogame expansion which took place in Rashemen. Though these sources may be pre-4e (the whole mystra explosion thing) lorewise, but should provide an adequate image of Rashemen no matter the age.