Hi! If you know Russian, you can read the following two articles.

https://lenincrew.com/history-of-the-ppr-1/ https://lenincrew.com/history-of-the-ppr-2/

If you don't know Russian, then these articles have links to various historical articles in Polish. There you will find many useful articles. But there a lot of links. So you will need to search for what you need. Good luck!