the best is the one made by google themselves Call Google Help https://adwords.google.com/support/?hl=en_US
Yep most Adwords book are crap, but there's a course called: Wholesale Traffic System. Created by Ryan Deiss and Perry Belcher ($100 million a year). I managed to slash my CPC almost in half by utilizing their techniques. The course costs $1997, and it's sold out. Email me at And I might hook you up with some links.
Having written a book on Adwords, understandably I think that some have value (including mine). Google's Help Centre is essential reading first. It will tell you how Adwords works (which puts you ahead of 90% of advertisers, and quite a few Agencies!), but in terms of using this information to maximise the profitability of your campaign, it's limited. Here's my free guide, if you're interested: http://www.adwordsprofessional.com/google-adwords-introduction In addition, I'd suggest reading the blogs of experts such as Dave (aka Cianuro) at Redfly (http://www.redflymarketing.com/blog/) and Jeremy (aka GuyFromChicago) at PPC Discussions (http://www.ppcdiscussions.com/).
How to Guarantee Failure Using Adwords is also very good. You can get it here for free: http://www.barracudamarketer.com/
Hi There is not any best adword books since the industry is so dynamic and changing our by our, no one can afford to write a best book. Though you can not find the best book, but you can get good information about it on google adword forums only