This takes on the wrong side of the closed e-reader problem, IMHO. There are lots of top notch devices available for cheap, but they have closed software. Howev

This takes on the wrong side of the closed e-reader problem, IMHO. There are lots of top notch devices available for cheap, but they have closed software. However they generally run Linux. Making a custom kernel/distribution for existing devices would be a better way to go. The Open Inkpot project tried this years ago, but never really got anywhere. It's low-hanging fruit though: compared to what projects like lineageOS are doing, this is very doable. And if this Open Book project ever gets usable hardware out, they'll still have to take on the software side.