1. Soulless, by Gail Carriger. I love the world that's created here-the humour in it is amazing and the mix of werewolves and vampires works well. The steampunk elements are great, and you can easily imagine the world.
2. Leviathan, by Scott Westerfeld. The world he creates is amazing, and I like the history re-done. The illustrations by Keith Thompson are brilliant too!
3. Clockwork Angel by Cassandra Clare. As well as making the Mortal Instruments something I want to read again, the world was imaginative and pulled you in.
And the things I haven't read but really want to...
Whitechapel Gods by S.M.Peters
The Girl in the Steal Corset by Kady Cross
Corsets and Clockwork AnthologyThe League of Extraordinary Gentlemen by Alan Moor
The Greyfriar by Clay Griffith
The Time Machine by H.G.Wells
And un-book related, but never mind... music
The Clockwork Caberet
The Clockwork Dolls
The Dresden Dolls
And random things that often get associated with Steampunk
Browns, golds and whites
And so on...