I've been playing with the SDK here:
https://github.com/pocketbook/SDK_6.3.0/tree/6.5
It was updated last year and the most up-to-date I could find. I've managed to cross-compile a few apps for the PocketBook successfully with it.
Very much a newb with it all, but with a lot of reading and a laptop with Ubuntu on it, it wasn't too bad. With git and cmake at the command line it can be a quick process. Finding missing libraries however... far less hair than when I started.
The pocketbook-demo
imustafin mentioned is definitely worth playing with. Some great examples and templates.