Is your book enrolled in KDP Select? Is your book available in audiobook format?
If you answered yes to either of these questions, a Reading Stacks feature is for you. Reading Stacks drives page reads in Kindle Unlimited and drives sales and redemptions of audiobooks on Audible, Audiobooks.com, Scribd and Kobo Audio.
Note: books and audiobooks must have 20 or more reviews in order to be eligible for reading stacks.