I really wish client side end to end encryption would become more widespread in the developer community. It is not hard to implement anymore and takes away all the headaches of having to comply with certain regulations.
There are so many laws now that regulate copyright and hosting. An accidentally public S3 bucket full of ebooks could technically land you in jail in certain jurisdictions. Automatic shutdown notices. Providers themselves may also automatically scan stored material, and so on. There are so many reasons not to host your media library unencrypted on a third party server.
I recommend using this kind of tool only if you can prove that you are legally allowed to possess every ebook you upload in every jurisdiction you travel to or live in, as well as in the jurisdiction where the server is located.
RIP Aaron Swartz.