Leviathan by Scott Westerfeld (2009)
As far as YA steampunk goes, it doesn’t get any better than Leviathan. It is a brilliant recreation of World War I, featuring all the clockwork vehicular awesomeness that you could ever want. It also happens to have wonderful characters and just a great story. If you’ve never read a steampunk book in your life and love young adult novels, start here.