Once a flipbook has been published as public, it cannot be changed to unlisted, as the flipbook is already public on the internet, and it might’ve been already indexed by search engines. However, there's a workaround solution for this specific situation, which will solve the issue for most of you.
We propose that you do the following instead:
1. Make a copy of the old flipbook.
2. When you choose to publish, make sure that you select the unlisted option in Visibility.
3. Delete the old flipbook.
The statistics from the old public will not carry over to the new flipbook.
Once you delete your public flipbook, its URL will return a 404 error code. So, if you had shared the public flipbook's link before, that link will not work anymore. Make sure that you resend the updated link, so people will be able to access the new unlisted publication.