The money you make from selling your eBook is only the tip of the iceberg when it comes to online profits. By finding the right affiliate products and services to startegically include in your eBook, you will end up making four or five times as much as the initial cost of your eBook. Even if people give away your eBook to others, you will still make money with it over time.
You will want to find affiliate programs that offer something that is complimentary to the subject matter you have written about. I like to include my affiliate links both in the chapter of my eBook where they make sense, as well as at the end of my eBook in a resource section. This gives people two opportunities to find out more about the products and services you are promoting.
Additionally, it is my personal perference not to recommend something I have not used and benefitted from myself. This way I can personally endorse the person or service I am recommending. For example, I encourage everyone to go over to Clickbank to search for appropriate digital products to promote to people on their list or in their eBook. I also just recently became involved in DealsDotCom, and suggest you take a look at what they have to offer.