Remarkable eink tablets. Buried deep in the settings they actually give you the root password so you can SSH in. Also, it comes with an epic .vimrc file.
But, the software they ship is completely closed source and displaying anything on the screen requires hacking the binary (each software release). They have have been the opposite of helpful to open source for the last few years and have stooped to a cloud company trying to collect your data.
What is the best supported Linux e-ink device that's decent? I really want the PineNote, but it seems like its hard to find in stock and its pretty damn expensive. At this point, I'd consider building my own with off the shelf parts.
https://github.com/reMarkable/linux
At that, there are ways to hack it of course (Fairly certain it “ruins” some returns/warranty policies, but if you can in those cases, you can easily revert the modified bits if needed)
Kindles too. You can jailbreak them and get a shell. They're so much more useful when they're jailbroken. They can read multiple other formats, they can get books from a fileserver on your local network, the jailbroken reader app is better, etc.