I transfer books by running `python -m http.server` on my phone or computer, then opening my Kindle’s browser to my IP and downloading my .mobi book. It doesn’t take long, and I can do it all over Wi-Fi.

I can mount it via SSHFS for anything more than copying a single book.

I stopped buying anything from Amazon on principal a couple years ago, books included; and anyway, most books I read these days are in the public domain – Project Gutenberg is a treasure trove!