Read
You Can Negotiate Anything by Herb Cohen. It's one of the best books on the subject of negotiating that you'll ever read.
The free briefs on
http://www.marketingexperiments.com/ can be helpful in developing a good mindset regarding testing and measuring. (Sorry it's not a book.)
The "Mister X" book by Jay Abraham has some insightful stuff in it that you can apply to an affiliate marketing career, even though a lot of it is aimed at brick and mortar businesspeople.