what are you looking for.
are you beginner? do you have some knowledge in other programming languages, maybe flash?
like you said, there are several books out there, so it depends on what you need.

for a beginner i mostly recommend the new book by casey and ben as it is not as big as most of the programming books but still covers most of the important stuff :
http://www.amazon.com/Getting-Started-Processing-Casey-Reas/dp/144937980X/ref=pd_bxgy_b_img_c

if you have some flash knowledge, the following book by ira greenberg might be good as well
http://www.amazon.com/Essential-Guide-Processing-Flash-Developers/dp/1430219793/ref=sr_1_1?s=books&ie=UTF8&qid=1289307829&sr=1-1

and another really good book, but huge with almost 1000 pages is
http://www.amazon.de/Processing-Visual-Artists-Expressive-Interactive/dp/1568817169/ref=pd_sim_eb_7
but it covers a lot of interesting stuff, from basic to advanced techniques