At the risk of sounding like I know what I am doing- I have been playing around with code in some of my WordPress plugins. Some of the Amazon plugin automatically embed Amazon products into your posts using Amazon API key and search index function. By default the category i in plugins is usually books but I wanted to change to be more specific- like electronics in posts about Blu-Ray players, etc.
After playing around here are the categories I cam up with- I have found the more tailored amazon affiliate links are to the post the higher the CTR and the better conversion to sales. I have an excellent conversion to sales with Amazon and it almost rivals my Adsense revenue at this point.
I am also very wary of relying to heavily on AdSense ( or any one single revenue stream) and happy that I have invested time with Amazon. It is not nearly as easy as slapping in some code on your posts but can be worth the effort in long run. It took almost a year of diligent link embedding in several blogs for me to see real progress and earn income worth mentioning.
List of Search Index Categories for Amazon AWS