I think the heart of Steampunk, as most people understand the word, is a vision of a milieu with the technological trappings of science fiction as Jules Verne (or H.G. Wells) might have written it. Hence, big steam-powered devices, clockwork technology, ether-ships, phlogiston cannons, etc.
But just as Bluegrass music has thrown up and embraced its heretics and deviates, e.g., Sam Bush, David Grisman, John Hartford, Jerry Garcia, Bela Fleck, so Steampunk, from its beginning, has included a lot of stories that are not just straight riffing on Verne, but are Verne+. Here are some examples, in no particular order. They’re all pretty clearly Steampunk; also, each of them has something else going on in it.
Scott Westerfeld has written the Leviathan series (YA), comprised of Leviathan, Behemoth and the forthcoming Goliath. It is Steampunk “updated” to the First World War, with the global conflict between Clankers (riding around in big steam-y machines) and Darwinists (riding and fighting with genetically modified creatures of various sorts).
Philip Reeve is the author of the Hungry City Chronicles (middle reader — the original UK title is the Mortal Engines Quartet), the first of which is Mortal Engines. The Hungry Cities high concept is very cool: in the distant future, cities are giant, mobile, steam-y machines, and the big ones get raw materials for their maintenance and expansion by chasing down and devouring smaller ones. It’s set in a distant and post-apocalyptic future and features some straight futuristic technology, like weapon satellites.
Stephen Hunt writes “Jackelian” books (books in a shared setting but not necessarily connected in plot). The first is The Court of the Air. These books are set in an alternate world that is sort of Dickensian Steampunk plus magic plus inner earth/underworld story. The Court of the Air features young protagonists but is written for adults.
China Mieville writes, well, stuff. See, e.g., Perdido Street Station. It’s Steampunk technology, but the technology behaves like cyberpunk tech — there are artificial intelligences, and characters with steam-powered grafted limbs, etc. There are fantasy races, but they’re bugs and frogs and birds. Also, there is magic, and a grotesque, ghormenghastly esthetic to everything, and cyberpunk/radical politics, and horror creatures and plots. Also, the main character is a city.
What’s in your Steampunk?