Philip K. Dick really kicked off this genre with The Man in the High Castle where Germany wins WWII. Harry Turtledove really became the flag-bearer of the current field of writers starting with his Guns of the South, (Time-travelers equip the Confederacy with AK-47’s) and his incredible series of books that covered the South winning the Civil War, through WWI and WWII (Picture, if you will, trench warfare cutting across the United States in WWI and you get the idea). Robert Conroy has stepped up recently as well with a number of wonderful stand-alone books set in WWII.
The Steampunk genre is really an off-shoot of alternate history. It provides a glimpse into a universe where advanced technology is introduced in the pre-Victorian era. While Steampunk stands on its own, it also has strong roots with alternate history…a relationship all parties encourage. Read Boneshaker by Cherie Priest and you will see an alternate world where zombies face zeppelins and new technology.
I have found that some mainstream historians don’t see the value of this kind of literature. They see it as trivial or in some way diminishing from "real" history. I respectfully disagree. Alternate History is most often built on hard historical study, with the changing of an instance or event to generate the story. This kind of literature actually encourages a study of history with readers – which is something that most historians desire. It gives us all an appreciation for how history is like a game of Jenga. Remove the wrong part and the entire tower comes tumbling down.
Alternate history historians like Giangreco's Hell to Pay, Operation Downfall and the Invasion of Japan, have given new context to decisions like the dropping of the atomic bomb. While some authors tell the stories of these events, we are now seeing books emerge like this where historians give us a solid tangible history of what might have occurred. I have a pet project I have been researching along these lines as well and find this kind of research exciting. It bridges between alternate history fans and hard-core historians.
If you have not explored this genre, I encourage it whole-heartedly. The stories have impact on our world today. If you read Turtledove’s Man with an Iron Heart on the surface it is a story about the Germans fighting on after Hitler’s death, waging a guerilla war against the occupying allies. The parallels between this and the insurgency in Iraq cannot be ignored or overlooked. Books like this force us to put current events in perspective, and in many respects that is the essence of what historical study drives to achieve.
If nothing else, you’ll enjoy the ride!