When it comes to an eBook reader, the choices are limited. The market is dominated by Amazon’s proprietary Kindle along with a few other options like Kobo, Nook and Onyx.
An interesting news for open source enthusiasts is that a developer, Joey Castillo, is working on creating an open
Except I’m dead certain you can buy screens a lot larger than 4.2 inches - the 6 inch screen behind the Kobo Clara HD is actually all over AliExpress and eBay, and it has a backlight and is not marketed for as a replacement for the eReader. Kobo just glues a thin metal plate and a battery to it for rigidity, but otherwise it’s identical. And there are a fair number of other screens you can find online in various sizes if you want to get them working.
The hard part is getting data sheets or reverse engineering them. But it’s far from impossible.