Ask HN: How to store customer's digital books securely

So, We automatically convert digital books and pdfs to audio which our customers can then show on their website (login protected). Our customers are paranoid and want us to make sure that their ebooks and pdfs are not stolen and/or shipped off overseas.

How do I make sure that team members and hackers do not steal the books? We are thinking of locking the books behind an API service so that developers and team members can access it in only through the service. But it does not seem enough.

What do you suggest? Are there any services which help with this?