Start with Harry Turtledove's "How Few Remain" and follow the series. Turtledove is a very, very good historian and this is an alternative history series based on the proposition that McClellan did not find GO 181, Lee was sucessful in the '62 Pennsylvania Campaign, and Britain and France intervened to force an end to the War and two separate nations. The agricultural South naturally fell into the geopolitical orbit of Britain and France. The industrial North, deprived of access to British and French markets fell into the German orbit. The rest follows quite logically. And it is all a rousing good read as well.