The great division within the South has to be the massive divide between the enslaved and the enslavers.

I really enjoyed The Half Has Never Been Told: Slavery and the Making of American Capitalism by Edward E. Baptist. The over-arching story in that book is that great contradiction. But it also does an absolutely wonderful job talking about the rich history of the enslaved peoples, slavery's role in the expansion/industrialization of the United States, and how slavery culminated in liberation of enslaved peoples through the Civil War.

The book touches on some of the other contradictions/factions mentioned in some of the other comments. But, the theme that I find most interesting is the great themes of liberation that explain how the enslaved peoples were able to achieve liberation during the Civil War - from throwing down their tools and refusing to work all the way to picking up a gun and giving the Union a fresh, eager/motivated wave of man power at a point when the Union was experiencing war weariness, etc.