The application was removed from Play Store. Nowadays, it's available only on various 3rd party store websites which aren't safe to install the software from. However, your books should be available on the internal or external storage of your device. To access them, use the built-in file manager and browse to the directory of the application's name or simply Books. You can recover the files only if they're on the external SD card. The internal storage can't be used for recovery purposes in case the files were deleted.
Install Recuva, then connect the SD card to PC using a card reader and start the recovery with the help of Recuva Wizard.