SO! On to the Leviathan series! It's basically a revisioning of WWI, but steampunk, so the opposing armies are an army with mechanical creatures against an army using gigantic animals (some of which are filled with hydrogen and are turned into organic airships!). He's created a world so much more complex and complete than most YA storyworlds, down to the very language that his protagonists use. All of their sayings relay culture, making the alternate reality he creates feel real. There are pictures every so often, which I thought was a little dorky when I first saw it in Leviathan, but then realized was necessary because of all the crazy contraptions and creatures he describes--you seriously need pictorial representations to figure out what the crap some of these things are supposed to look like. But soooooo cool.
The books are very episodic, and there's so much going on in each episode, it was a little harder to feel hang on to larger storylines going on. But the characters and their motivations and feelings are always clear. I'm always a fan of too much complexity rather than too little. Reading this book felt SO different from the normal YA fare--Westerfeld just takes it to a completely higher level on all accounts. Reading the book was like a mini master class on how books should be written. I'm trying to hijack some of his methods for culture building to make the storyworld in my novel fuller and more realistic.