I think you need to define some time-scales here. Are you looking at songs in 2011, or at the evolution of sentiment into song at the creation of 'country' (however you define that), or are you looking at the entire evolution from the time of the Civil War into the modern era?

As background, I found E. Lawrence Abel's "Singing the New Nation: How Music Shaped the Confederacy 1861-1865" (2000, pub.Stackpole Books, Mechanicsburg, PA) to be a very good analysis - it also shows how sentiment gets amplified through popular song. Could be a good starting point!

Good luck with it, whichever way you're doing it.

TomB