Many authors like using Amazon Associate links not only for income but to track which promotions work successfully and which do not. This leads to use Amazon Associates links with books which are on free promotion.
However, Amazon operating agreement penalizes users who promote free books and to enforce this rule Amazon does not pay ANY affiliates fees for the entire month when this rule is violated.
So what is this rule exactly? Keep in mind that this rule was invented by Amazon to stop various website publicizing free Amazon e-books. I am not sure why they think it is a bad idea, but we are here to look at the rule not the reasons for it.
Below is the section of the Amazon agreement that addresses this restriction. It states that if your links refer people to order more than 20,000 free e-book purchases in any given month and this constitutes more than 80% of all Kindle eBooks attributed to your links then the entire amount of your Amazon Associates fees will be withheld that month.
This is bad news for a web site that generates a lot of traffic, but for an author that posted a link to a free promotion of his/her own book and sold under 20,000 copies this rule is nothing to worry about.
Here is is a copy of the complete rule
Free Book Promotions
You will not be eligible to receive any Standard Program Fees or Special Program Fees for any month if we determine that your Site is primarily promoting free Kindle eBooks and during that month (i) 20,000 or more free Kindle eBooks are ordered and downloaded during Sessions attributed to your Special Links, and (ii) at least 80% of all Kindle eBooks ordered and downloaded during Sessions attributed to your Special Links are free Kindle eBooks.