> the Mississippi River Valley had more millionaires per capita

TIL.

Does anyone have any good recommendations for a (relatively) good social/economic history of the Southern US states? (because I guess that that type of history book would cover this type of information).

from the antebellum era? try anything by eric foner, but a good place to start is forever free: the story of emancipation and reconstruction. it's accessible history that includes a bit of background about the political economy leading up to the civil war. any of his books about reconstruction might be interesting but not exactly what you're looking for. "black reconstruction in america" by dubois is a keystone text that has a bit about the southern economic history. and maybe "old south, new south" by gavin wright for post reconstruction economic history