The 20th century would be the postbellum South. That being said, there has been a tendency to view the antebellum south (the South until the Civil War) as a cultural backwater/wasteland too. This work by Michael O'Brien which shows that the Old South was actually well connected to the rest of the Atlantic world.

As for books about the Postbellum South, C. Vann Woodward's Origins of the New South might be a good place to start but that deals with the 19th postbellum South primarily. I'm not as strong on the 20th century but James C Cobb's Epitaph for the North could also be a good place to start. In that article he discusses how America has been "Southernized."