I've always been a fan of the alternate history book series, beginning with the "What If" series in marvel comics, going through to television shows like Sliders, Quantum Leap, Early Edition and now Journeyman, but 3 years ago I was pleasantly surprised by a guy who made his living off of Alternate History named Harry Turtledove.
How Few Remains starts off with a few pages of prologue showing where the North Went wrong during the civil war, and how the South ended up winning it. The rest of the book is a second civil war twenty years later. The cast is still the same, except this time they all have different parts.
For instance, Abraham Lincoln was never re-elected, and got soundly defeted, prompting him to turn socialist and not really have much of a say in politics.
Books like this make me excited and I just love to go around reading what COULD have happened.