In Calibre, you can also run a web server. On her Kindle Paperwhite, my wife simply goes to the "experimental web browser" and browse to 192.168.....:8080, which shows all the available books. From there she downloads the mobi version of the book she wants and done.

I'm actually a big fan of rtorrent/rutorrent... https://github.com/crazy-max/docker-rtorrent-rutorrent