The obvious innovation is to just make the back of the phone case an eInk display. No need for all the bulk when you can merely have an app on your phone that controls the case-display and the phone can output whatever reader app you want to it like a companion/IoT device.
Or it can be a little bit bulkier and just be a dedicated ereader that is shaped like a phone case. Either way works.
Or it can be a little bit bulkier and just be a dedicated ereader that is shaped like a phone case. Either way works.