The Warlord of the Air by Michael Moorcock (1971)
Our friends across the pond know Michael Moorcock as a household name in sci-fi. He may not be a hugely popular stateside, but his influence in the early days of modern steampunk is pretty serious. The Warlord of the Air is a great example of the early formation of the genre, although with Edwardian flavor rather than Victorian. Future writers would tweak the formula a bit but in The Warlord of the Air you can really see the foundation starting to form.