This popular sub-genre of science fiction imagines what the world would be like if history had turned out differently.
What if Nazi Germany had won WWII or Great Britan defeated the Colonies in the American Revolution?
What if Lincoln had survived or Alexander the Great died prematurely?
To find alternative histories in the lbrary catalog search for
11/22/63 by 
Bring the Jubilee by 
For Want of a Nail : if Burgoyne had won at Saratoga by 
One Lamp by 
The Best Alternate History Stories of the 20th Century by 
The Guns of the South by 
The Two Georges by 
Virtual History by 
World's Foremost Military Historians Imagine What Might Have Been by 
The Man in the High Castle by 