I think Google Advertising tools by O'Reilly is my favourite, although I am not signed up for Adwords.
Perry Marshall's guide is quite good as a starting point. I think it's worth the money...not sure if links are allowed here but the site for his book is perrymarshall.com/adwords
I'm not the biggest fan of Perry Marshall's book - he focusses on advert text and clickthrough rate (which are important), but barely mentions the conversion rate, or the Quality Score (which are far more important).
Wow, CustardMite has a pretty darn good site there. Easy to read and easy on the eye. Good job Custard. Me likes. Plus its FREE?
I like the Beating Adwords eboook. Before I read that I liked Google Adwords 123 but that was when I was now getting started.
I think anyone starting out should just look at the Adwords training. There's a ton of FREE information in there, most e-books just repost that information but put some real life examples in and say how they made X CTR on this ad and X CTR on that one. Besides the free training just test it on some low traffic keywords. Try product names that are low competition but still get 5-10 searches per day.
I agree to some extent but for someone who knows almost nothing about adwords I think it's a pretty good starting point. If I remember correctly I think it's a bit outdated, maybe before quality score etc were implemented? Anyways I found it interesting, but moved on to more advanced books after. (But I'm pretty new to all this so what do I know )