Kobo offers a complete ecosystem similar to the Kindle, with bookstore, devices and syncing. There are a number of regional players as well in this space.

Then, of course, there are all the device makers that simply expect you to bring your own content.

Someone on hacker news talked about using syncthing for their ebook and I had to replicate it. it sounded so much more useful than my previous experience with ebook readers, the nook simple touch.

My quest to find an e-reader that supported Android 4.4+ and both headphone jacks and SD cards resulted in a likebook Mars.

Now I can use syncthing to sync my library...ebooks, PDF, and audiobooks, automatically in the background, no account needed.

I would have preferred to purchase a modern incarnation of the very first Kindle, with a headphone jack, SD card, and keyboard, but nobody makes such a device any longer, or if they do, it is so out of date that it cannot run syncthing.

I continue to be excited about the pine phone keyboard design, because from there it is only a properly sized e ink screen away from being a tiny e-reader with keyboard headphone jack and Linux!