Oh, this will be very useful. My current solution is incredibly hacky, I run an unauthenticated SSH server on the Kindle (key-based wasn't working), port scan to find it, and SFTP new files. At home, at least, I have a static IP. The whole system falls apart enough that I usually just connect to calibre's remote server and send books that way, though. I wonder what the battery impact of running tailscale on a Kindle is.