Jum... not my user case, I don't know any plug and play, but in case you want to research a little, I think that https://calibre-ebook.com/ and https://syncthing.net/ could be a good match. The first one is a power house to manage ebooks, you can even access in your local network from the phone, maybe there is some config (vpn/vps) you can use to be accessible outside your network.

The second one is a general purpose sync program to maintain directories in sync.