What e-readers:
* work with a relatively non-proprietary linux distro (e.g. debian, suse, not android)
* have a screen that is e-ink, reflective LCD, or dual-mode (but not strictly backlit LCD)
* are mostly open (i know nothing is fully open)
Anything that is heavily proprietary suffers from making users dependent on the maker for updates. If a maker wants you to buy their next model, they can simply drop support for old models. E.g. nook and kindle users will be screwed in such an event. Whereas if debian runs on a device, there is at least a reasonable chance that the FOSS community will continue support.
Kobo looks viable, because
debian has been ported to Kobo and
Mer has been ported to Kobo. And there is an open
git repository for Kobo. So if the makers of kobo drop the ball, others are likely to pick up the slack.
Is there anything to consider besides Kobo devices?
In principle, I would rather reward Onyx and PocketBook makers with my business, because they don't try to force a compulsory registration on their clients.