KDP updates bonus content rules.6595
Kindle Direct Publishing has limited bonus content to 10% of ebooks in an attempt to prevent authors from gaming the page royalties payment system:
If you choose to include bonus content (e.g., other stories, or previews of other books that are not part of your book’s title), it should be relevant to the customer and should not disrupt the reading experience. To meet these guidelines, we require placing additional content at the end of the book, and listing the bonus content in your book's table of contents.
To provide an optimal customer experience, bonus content should make up no more than around 10% of your book. If you would like to include multiple stories within your book, consider creating a collection of works. When selecting your book’s title, always make sure to follow the Metadata Guidelines.
Primary and bonus content must meet all program guidelines (e.g., bonus content in KDP Select titles must be exclusive). Translated content must be high quality and not machine generated. Disruptive links and promises of gifts or rewards are never allowed.
For more information, see our content guidelines and Terms and Conditions.