KDP Select is a free and optional program available to authors and publishers using Kindle Direct Publishing (KDP) to publish their Kindle e-books. It is available on a book-by-book basis and only applies to e-books.
If you enroll into KDP Select, it is a mandatory 90-day term stating that you must be exclusive to the Amazon Kindle store and therefore cannot sell your e-book in any other store, including your personal website. It also stipulates that you cannot give your e-book away for free.
Is KDP Select worth it?
If you’re a new author and your book does not have a large or broad existing readership, then KDP Select is for you. Also if your book is classified as genre or popular fiction, then KDP is most suitable for you. Remember that this exclusivity requirement is only for 90 days, meaning that you can test the waters and unenroll if you find that it’s not benefitting your sales.
Bottom line is that Amazon is by far the largest e-book retailer in the United States so depending on your niche and market, and who your book appeals to, keeping your book in KDP Select is likely more beneficial than pulling it out and selling it on other platforms like Apple iBooks or Ingram distribution, who has a smaller market.