Alternative History genre is among my favorites. It is just that: one or several historical facts are completely or partially changed in the book, and whole world is then perceived via the prism of that possibility. Steampunk genre is most commonly known in Alternative History genre, since it operates on alternative possibility of Steam Engine being the one that got developed wider and longer than it actually was.
My most favorite examples of Alternative History could also be called Alternative Reality, since fantasy enters (fantasy as in supernatural, not as in mere fiction). One being Anno Dracula by Kim Newman. We all have read Bram Stoker’s Dracula, haven’t we? Or at least seen the dozens of movies that we can choose from without even looking for them (and if you do look, there’s hundreds awaiting). There the brave men ganged up and killed our Father in Darkness (in Anno Dracula that is how your sire is called, Mother/Father in Darkness). Yet in Newman’s story that is not what happened at all. Some meager little men against someone like the Impaler? Ha. He rips them to shreds, in some cases even literally, and slowly but surely brings vampires into the daylight. Not literally. History proceeds as is, but vampires are a great part of it. Say, Bloody Red Baron tells the famous Red Baron story, yet here we not only have some epic pilots doing their jobs, heck, we have giant blood thirsty bat-dragons.
The other example is Bartimaeus sequence by Jonathan Stroud. Empire of Great Britain has never crumbled, but just went on, and on, and on, becoming the greatest power since Roman Empire if you please. History ceases here, since this is not the main genre of the book, and yet it fits the frame.
There are, of course, many more, since I told you what Steampunk is at the base, so even our very own Tapinas with his Wolf Hour is guilty of being Alternative History writer. It’s a genre for all those who overthink things, I think (see what I did here?), since it allows us a wider view of what could’ve been, and how that would’ve changed our lives as we know it.