On the Overview page for your book, in the "Community" section, you can find links to a "Customers" list and an "Interested Readers" list:

he "Interested Readers" is a list of people who sign up before you publish your book for the first time; they will be notified via email when you publish the first version of your book.

The "Customers" list is a list of people who actually buy your book after you have published it, and who have also opted in to share their email address with you.