I have not read these personally, but worth a look:

http://www.amazon.com/Covert-Java-Techniques-Decompiling-Eng...

http://www.amazon.com/Decompiling-Java-Godfrey-Nolan/dp/1590...

Venners book has free chapters online (be careful this is pretty old):

http://www.artima.com/insidejvm/ed2/index.html