Many affiliate programs rely on their novelty to make the sale.
In other words, if you're the first person to show potential buyers a certain offer, they're much more likely to make a purchase than if they had seen the offer 100 times before. Book affiliate programs are different. And ezines are a great vehicle for doing this. They may be regular customers of these online stores, and have come to trust that they'll be receiving a consistent and good level of service when they purchase from them. By providing links to products they may want at stores they're probably already familiar with, you're using the popularity of these stores to your advantage.