I enjoyed Marcus Cunliffe’s “What If?” but I notice he stuck to the academically “safe” examples. Generally regarded as the best speculation on a world where the South won the Civil War is not the Churchill essay but Ward Moore’s novel, Bring the Jubilee , which is less optimistic and more realistic than Churchill’s scenario. And I’m surprised that he omitted Robert Sobel’s For Want of a Nail (1973), a largely economic history of British North America following the crushing of the rebellion by Burgoyne at Saratoga.
Also interesting historically is L. Sprague de Camp’s short novel, The Wheels of If , in which the crisis points are the Synod of Whitby (which I had to go look up after reading the story) and the Battle of Tours, plus having the Viking contacts with America continued and Vinland being a modern nation. De Camp even goes into linguistic changes, a rare thing in alternate histories.