Any Ereader with Linux?

Is that an EReader running on a Linux desktop, or an EReader running Linux as its OS?

Calibre* runs on Linux, and it's a complete ebook library app as well as having an ebook reader built-in. I have several thousand ebooks being managed by Calibre.

The Kindle is an ebook reader that uses Linux as its OS.

* This is my 'update.calibre' script:

    #!/bin/bash
    #
    sudo -v && wget -nv -O- https://download.calibre-ebook.com/linux-installer.sh \
     | sudo sh /dev/stdin

Pretty much all of them are actually running linux, but are also proprietary. You could try the Pine Note, but Kobo and Pocketbook are reasonably open as far as these things go. These devices are mostly disposable though due to the designed-in fragility, so I wouldn't bother obsessing over the software - you'll need a new one in a year or two.

Does this suit your requirements:

https://www.pine64.org/2021/08/15/introducing-the-pinenote/