Due to the fact that I know practically nothing about eBooks, and that I am very interested in the business aspects of how they work, the following articles was very interesting. The article Creating Additional Value from eBook Sales by John Thornhill, discusses how Affiliate Marketing comes into play with ebooks to make them more profitable for the publisher.
Electronic links, also known as Affiliate links, are sent to the consumers of the eBook that was purchased, through a number of direct marketing schemes. The links correspond to sites that cover the same topic as the book that they purchased, and are specially coded so that you, the publisher, get a commission of something bought off the affiliate site. Many manufacturers have affiliate programs available, and these links can also give the original eBook greater value in the long run. If you are "self-authoring" an eBook you can just include the affiliate links in the book, without all the direct marketing issues.
Including these links can be very profitable when used efficiently. However, the over use of affiliate links can cause the eBook to look like pure advertising with no content, so it is suggested to use them moderately and efficiently to keep the probability of a visit to an affiliate link high.
More or less, in conclusion, affiliate links in an eBook can boost the value of the book, and increase profit even after the initial sale is over.
http://articleebusiness.com/ebooks/creating-additional-value-from-ebook-sales.htm