At Leanpub, we strongly believe it is a good thing to encourage people to sign up so that they can be notified when a book is first published!
We set this option up automatically on the landing page for every Leanpub book, when any new Leanpub book is created.
Here's what the "Interested in this book?" feature looks like for an unpublished book:
After people start signing up, you can see some information about people who sign up here:
...making sure to replace YOUR_BOOK with your book's unique web address.
You can also find a link to this page from your book's Overview page: