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.
Yeah but I’ve seen color e-ink fridge magnets that are like $30. I don’t think it ends up being more expensive or substantially worse for the environment than people buying cases made of fancy leather. Presumably the display module could also be cut out of it for DIY uses.
Or it can be a little bit bulkier and just be a dedicated ereader that is shaped like a phone case. Either way works.