Reddit reviews Underground Airlines

There's actually a great book I read recently called Underground Airlines that deals with this sort of alternate history. In short, the Civil War never happened, as a compromise was struck in order to preserve the Union. A bunch of constitutional amendments were struck outlawing the federal government from banning slavery, but states were free to ban slavery. In the present day, four states still allow slavery, and the USA as a whole is a shunned, backwater country because of it.

Anyone interested in this series may want to check out Underground Airlines. Reading it now and it's pretty solid. It's set in modern times but essentially Lincoln is assassinated in 1861 and the Civil War never happens.