Alternate History Sci-Fi: the Best Earths that Never Were

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.

Dinosaurs and humans coexist, but not the way creationists envision it. The dinosaurs never died out; humans still evolved, but are a subservient species to the dinosaurs. You'll recognize the continents on this alternate earth, but recorded history started much earlier! This is probably the longest-ago turning point of any alternate history I've ever read; I don't think one could go back much further and still recognize it as human history on earth!

The turning point for this alternate history is millions of years ago: the Mediterranean Ocean didn't fill in after a bit of continental drift, and Neanderthal man never died out and coexists with homo sapiens, in a civilization that has developed with some of the same technology ours has - it's set in an equivalent to modern times.

From the golden age of science fiction, this book is one of those that shows that there's a rather fuzzy line between alternate-history and time travel, and sometimes one can cause the other.

One of the greats in the "time traveler alters his own timeline" style.

The short story that started the "butterfly effect" name for chaos theory, this one is another classic in the "time traveler alters his own timeline" area.