my files gets leaked

Hello Everyone, I need your help. I run my website on wordpress which is about my ebooks. But unfortunatley hackers are able to download those ebooks without going through legal steps I.e making payment. they find my files on WP-contents and some times they are able to find my website registration link. Please tell me how can I make them secure by hiding my ebooks from wp-content and how to hide my registration link or at least only those can be able who has made payment. Thank you