One of the first "what if the South won the Civil War" novels, and still one of the best. Later alt-Civil-War stories get most of their ideas from this one. Our hero starts out as an apprentice in the impoverished north of the post-Civil-War America, but finds himself drawn to an unusual community of intellectuals and scientists, who may or may not be able to illuminate the decisive battle that allowed the South to win.

Although there were what-if-the-Germans-won stories that came out right after World War II, this novel from the early 1960's was one of the first full-length novels written by a science fiction author. It has a slightly hallucinogenic quality to it sometimes. After Philip K. Dick, other authors have written alternate-World-War-II novels, some of them with far too many sequels, but this remains one of the best.

From a "literary" author rather than one of the usual SF suspects. What if the Protestant Reformation never took place, and the Catholic church remained all-powerful into the 20th century? Including the part where boy sopranos get castrated to keep their pure boyish singing voices?

I finished The Yiddish Policemen's Union in two sittings, one of which lasted 'til far later into the wee hours of the morning than it should have. Yes, it was that hard to put down. Yes, it is that good. It's not perfect, but it's damn good writing, and off the scales on plot and on novel presentation of interesting ideas/interesting presentation of novel ideas. Definitely worth it; it's a little bit depressing in parts, but it's also very funny, and I think it will become a classic so rapidly that you might as well add it to your classics list now.

A longer-ago divergence point than many alternate histories, this one takes Corporal Calvin Morrison, a PA state trooper, to the same physical area as Pennsylvania, in an alternate world where Christianity never arose, and where gunpowder is controlled by a primitive religion. Sort of a Connecticut Yankee in King Arthur's Court, only earlier.