What exactly is wrong with reading from SD card?
It’s not a limited or uncommon resource.
Near enough everybody has an SD card and people would just be using the SD card to flash the books onto their Pokitto anyway, why not save them the trouble of flashing?
That’s going to multiply the workload by the number of platforms you want to support.
Besides which, instead of doing that you’d be better off coming up with some kind of API that exposes the features of each platform you want to support in a unified way.
That way you could write the API and its implementations just once and then anything you want to port to multiple platforms you just rewrite to target that API.
But as this is the Pokitto forum, that sort of thing is almost certainly out of scope here.