I use Pocketbook app but haven't had any problems myself, so the following comments are just as a user (most use is on a Samsung tablet).
1. If you uninstall my experience is that you lose everything unless backed up.
2. I have not lost anything moving this app and its data (or any other app) to the SD card. Backup if unsure.
Pocketbook developer's contact is
info@obreey-products.com.
As your are probably aware, Pocketbook creates its own backups (and I think the default is for it to automatically back up?). If you want to be doubly sure then I use MyPhoneExplorer which is free in the store (if you decide to use it; under menu item Extras select Create Backup, follow instructions and it will eventually present check boxes so you can select the application(s) and other files you specifically want to back up i.e. it doesn't force you to back up all the applications, just what you want). Like all backups, nothing is guaranteed but this app has been around for a long time and is well regarded; however, I have not yet ever needed to restore anything from its backups. It does a lot of other useful things too.
John