No doubt clickbank.com, they have over 500k products, most of them are e-books. Also good is paydotcom.com.
If you like to promote e-books, the best for you would be to buy a PLR from theplrstore.com or any other place for about $5, then just promote that e-book you bought and keep all the money from sales
Amazon is a retailer for physical goods, not for digital products. There are no ebooks on Amazon. Furthermore, Amazon pays you around 5% per sale, meaning you earn only a few dollars from a sale, while with clickbank most vendors offer a 75% commission, so in most cases, you earn at least $20 per sale, higher priced items can bring you $50+ per sale
Overall, would have to agree with clickbank and second that is amazon for the e-books affiliate program