I'll add another vote for Leviathan, and also suggest the Anno Dracula series by Kim Newman. They aren't really steampunk (apart from the first two being set in the right era, and having a lot of characters from Victorian literature scattered throughout), but they share many of the same aesthetics and they're really, really good books. The second one, the Bloody Red Baron, in particular, you really can't top giant bat things tackling enemy aircraft in mid air. The third one also has some clockwork ballerina assassins and Frankenstein's monster, so you got a touch of Steampunkishness there.
A quick warning though, they are pretty gory books, and Kim Newman is almost nausea-inducingly good at describing scenes. The first book is basically the story of Jack the Ripper's rampage, but taking place in a world where Dracula won, married Queen Victoria, and turned her and a good chunk of England's population into vampires. Then some nut starts carving up vampire prostitutes. The second book has all the carnage of the first World War to top it off, and although things die down for the third book (set in the 1960's), it still has its fair share of blood and mayhem.
