amazon's commission is super tiny compared to clickbank. but then again, amazon is also having their own PPC campaign that spreads to any product under the sun. you'll 'love' the competition
Clickbank, most of the people follow it. Another option is that you can purchase an ebook with master resale rights and start selling it yourself using payloadz.
As an affiliate though the key thing you may want to know is that because each copy has minimal costs to the author, because we are talking about downloads here, the average commission paid can be very high indeed.