I feel, the BQ Cervantes line of readers are the most "open" devices.
The UI is built over QT and source available in github.(incl kernel).
https://blog.bq.com/es/bq-ereaders-developers-program/
This probably beats even the Irex Iliads.
But in spite of these, there seems to be very little interest in these devices.
Out of the box the reader seems to be quite bland and they are only in 6' size. Begging for some developer's TLC.