I read this somewhere around 2006-2008.
The book was about a kid (let's call him "Bob") who finds out he's the emperor of China. He lives with some poor old guy who is secretly some kind of master of a mystical martial art. This old guy gets bitten by a dog on his foot. Bob gets help from "friends" who give the old man some kind of poison (gin). This saves the old man. Then the old man fights some of the evil king's forces to protect Bob, and Bob escapes.
Bob acquires a valuable blade that needs to meet with its opposite half every once in a while to recharge. (Yanglao and Zhizheng?)
Bob's father turns out to be the evil king (of the Mongols?), and he's the "Yin" to Bob's "Yang". He possesses the counterpart to Bob's blade. He kidnaps someone Bob knows (a girl?), and threatens to feed them to some giant sea monster (a lobster?) if Bob doesn't show up. In the end, Bob fights the lobster by heroically flying, and wins, and throws a valuable blade inside the monster.
The back of the book compared it to Crouching Tiger, Hidden Dragon.