Where to do I find my saved book files on my root device?
First go to settings->Apps->Your app->move to SD Card. Then follow these steps:


First download ES File explorer
Search for the app's folder on the SD card.
If you cannot, go to the Android folder->DATA->Your app.
                  OR
go to Android->OBB->your app.


If you cannot find the folder, the developer ought to help you, especially since you are helping him.