title_body: There are a standard EPUB version or format to assign with PGP? There a standard way to assign (PGP) EPUB eBooks? 
  
  A way to use digital certificate, like in the PDF's certificate-based signatures, but not PDF.
upvoted_answer: Theoretically, you could add a signatures.xml file to the epub. However, I don't know of any epub app that actually supports this feature.
  
  I.e., you'd also have to provide readers with a standalone app that validates the checksums.
  
  It'd probably be easier to generate an MD5 checksum for the whole epub.