Looking at your site, the e-books are “protected” using a form forcing users to sign up to your MailChimp email list of they want to get access to them. Taht amek sit even harder to know, because the URL’s could now be set in your WordPress site, or in your Mailchimp account.

That’s the problem. No one here has access to your site to see what’s actually going on (and please do not give access to anyone off these forums, that’s a very bad idea!). If you need to know in a hurry I’d suggest hiring someone that can look through your site and it’s code help you to find out what’s going on.