You can also run Syncthing on a jailbroken Kindle. That opens up a world of possibilities!
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I have calibre set up to just email books to my Kindle, but that's an extra layer of indirection that I really don't need. I'll have to check that out.
Although, I realize Android != Kindle's OS, so I'm not sure how much concern there should be.
[0]: https://news.ycombinator.com/item?id=46184730 "Syncthing-Android have had a change of owner/maintainer"