I have the app and I was still able to read my books yesterday. Today when I opened it my library was empty. Is there any method to get my books back? I presume they are in my phone (Samsung s4) somewhere.
Your files should be located on the phone's storage. Install a file manager or use the built-in one and then manually look through the folders until you see the PDF files. If they were deleted, then recovery won't be possible since there is no tool created to recover data from partitions used by your phone. I recommend ES File Manager to easily browse or Search for those files.
You can use a data recovery tool to recover them.
Have a look at this guide,
recover android files
retrieve deleted text messages android.
You can follow it to recover your photos.