In the American Civil War, Confederate General Lee's Special Order 191 fell into Union hands, providing Union General McClellan with the location of the Army of Northern Virginia. This allowed for Union victory at the Battle of Antietam which provided President Lincoln with the proper conditions to issue the Emancipation Proclamation, making the Civil War a war against slavery. This prevented France and the United Kingdom from recognizing the Confederacy.
This is a common point of divergence in alternate history fiction. On its own, the rebelling states did not have a chance of victory if the Union chose to fight until victory. Their only real chances was to either convince the Union that victory was not worth fighting for or to secure foreign assistance. Harry Turtledove posited in this series that if the orders did not fall into Union hands, France and the UK would recognize the Confederacy, forcing an end to the Civil War.
The first novel in this series, How Few Remain, featured the Second Mexican War. In it, the Confederacy purchased the Mexican states of Sonora and Chihuahua from Emperor Maximilian (a European puppet who in real history was forced out of power for a variety of reasons, including that of the reunited USA not being very happy with him). The USA declared war in response to this expansion but again found themselves alone against a Confederacy with strong European allies. It also featured a still alive former President Lincoln, blamed for the loss of the Confederacy, splitting progressive Republicans out of their party into a Socialist party.
While How Few Remain featured historical characters as viewpoint characters, starting in the Great War trilogy, the viewpoint characters became purely fictional, though they often interacted with historical characters like Woodrow Wilson, Theodore Roosevelt, Al Smith, Louis Armstrong, etc. After the conclusion of the Great War, Turtledove quickly covered the 20+ years between wars, with analogs of the Great Depression and an American version of the Nazi party taking over one of the American governments, complete with a Holocaust on American soil.
Over the past few months I've been rereading books in this series. In all honesty, I'm going to have to describe the writing quality as... ok. There's a lot of repeated facts and descriptions. I think the term "zinc oxide" is used a few hundred times as we are treated to repeated descriptions of a fair-skinned American sailor's battles with sunburn.
With that caveat, it's a series with some pretty awesome ideas. It shows both the United States and Confederate States becoming more entangled with European alliances as they jockey for power - with some predictable results upon the assassination of Archduke Franz Ferdinand. I'm staying a bit clear of spoilers in this discussion, but we learn at the start of the Great War series that Woodrow Wilson is indeed president - of the Confederacy - with Theodore Roosevelt as president of the USA.
In this timeline, the Republican party is a very minor power in the USA, with a strong Socialist party taking their place against the Democratic majority (of which Theodore Roosevelt is a part). The Confederacy is not able to hold onto slavery for very long due to the objections of their European allies - however black Confederates are not citizens, they are "legal residents" with far fewer rights than the ruling whites. Under this backdrop, the oppressive whites of the Confederacy find themselves dealing with a "black red" revolution, as many of its black residents embrace communism.
The USA is less oppressive towards blacks, but that isn't saying much. Many whites in the USA blame them for the War of Secession (as the Civil War is called). Blacks attempting to flee to the USA are typically sent back to the CSA.
Border towns and cities prove interesting places - in several novels, Covington, Kentucky is a major location. It is a Confederate city across the Ohio river from Union Cincinnati, Ohio. It is a major battleground and changes hands multiple times, with numerous underground organizations.
One of my pet peeves with RPGs set in the Old West is they so frequently resort to having a victorious Confederacy. Sometimes this is done with a painful ignoring of many of the historical facts of the Confederacy. I really like the Deadlands RPG but I find its "and just like that the Confederacy decided racism was bad" to be extremely jarring. However, Turtledove's portrayal of the two nations strikes me as a lot more believable. The Confederacy is extremely dependent on both having European allies and Union ineptitude. By the 20th century, the Union has its own allies and has become extremely militarized. The novels do not portray every white in the Confederacy as a raging racist, but it does portray it as a racist government - and aren't much kinder to the United States.
One thing the novels do really get across is how fortunate the United States is to have emerged from the Civil War a single nation. It's difficult to imagine history flowing so similar to our own with such major changes, but it is difficult to imagine those nations as being friendly with each other. But as a thought exercise I find it quite compelling. As far as if one would enjoy such reading such novels, I find people tend to discover early on if Turtledove's writing style works for them.