title_body: Kindle Direct Publishing: How to mention other Books
upvoted_answer: Great question. I don't see any documentation for this in the Kindle guidelines. https://kindlegen.s3.amazonaws.com/AmazonKindlePublishingGuidelines.pdf
  
  My best guess is that this is simply an external hyperlink which Amazon recognizes, something like this: https://www.amazon.com/gp/product/B01EZ4B0UK/
  
  I'll test this on my own ebook and see whether I can duplicate it. 
  
  Finally, I don't think this feature is particularly useful. Having this book-buy link can be distracting.(I try to minimize the external links in my ebooks).