A libre equivalent to Kindle Reader

https://www.pine64.org/pinenote/

This, but it's not quite ready for release yet and also it's per se an e-reader but rather a tablet with an e-ink display, which makes it more powerful but also could have its drawbacks.

https://github.com/joeycastillo/The-Open-Book

Then there's this which is designed so that you build it yourself and to be relatively cheap, but you have to assemble it so if you don't have the technical skills it might be an issue.