A 1953 alternate history and time travel novel by American writer Ward Moore. Nowadays considered one of the English-language AH classics.
The novel is set in a world where the Confederate States won the American Civil War and semi-defeated the Union, making it a marginalised country in further history. The Confederacy becoming a major power has not made this world a particularly pleasant place, though, making it not only socially but also technologically somewhat backwards. At one point, the main character of the novel gets the opportunity to time travel to the Civil War's Battle of Gettysburg, where a major twist in the narrative occurs…
The novel's entry on Uchronia.net
The novel's entry on Wikipedia
Map of the world of Bring the Jubilee (in 1953), courtesy of B Munro