For me, Perry Marshall's Definite Guide to Google AdWords is, hands down, the best eBook on the subject. Perry's other products on AdWords also kick ass.
Perry knows his stuff. He has specialized in AdWords since Google started the program. He is a no-nonsense business man. He takes teaching seriously. He doesn't over-hype. And his knowledge is solid, tested, based on measurable results and immediately useful.
His products are also reasonably priced.
And, if you want to go the black hat method, you can find most of his stuff for free. Just do some poking around.
I've been through tons and tons of AdWords products and nothing has ever come close to his stuff.
I'm not affiliated with him in any way. Just a big fan.