I don’t see a mention of PocketBook so here it is, last time I checked they are running a linux kernel and the source is available and the device should be moddable/hackable.
I am in love with my PB Touch HD 3, does exactly what it needs without any annoying stuff (but with goodies like backlight and blue filter). I did opt into using their cloud for book syncing (which is not required at all, usb cable works too or other clouds) but there was never an ad or intrusive thing, love it.