Do you play nice with KDP Select / Kindle Unlimited terms?
Amazon's KDP Select / KU exclusivity terms state the following:
Have exclusive rights for the primary content of the Kindle book (i.e., the content is not public domain) - Source
If your book is in KDP Select / KU, we ensure your book does NOT appear in the public domain on BookSirens. For example, your BookSirens book page will not appear in search engines like Google, Yahoo, etc... Furthermore, readers would need to register and login before they can access your title.
We believe this precaution makes our service safe for KU authors. That being said, this is only our interpretation of the guidelines. So, please use review services like BookSirens at your own risk if your book is enrolled in KDP Select / KU.
We are happy to report that we haven't faced any complaints so far (and we've promoted over a thousand books enrolled in KDP Select / KU over the last three years).
Books on Preorder
Even though you may have enrolled in KDP Select for your preorder title, note that the KDP Select / KU term begins after your book is released.
You can enroll your pre-order book in KDP Select, and the KDP Select term will begin on the book's release date. - Source