title_body: How is an epub structured internally?
upvoted_answer: EPUB is an open format so you can find the standard specifications online. Wikipedia has a good article on the EPUB format. 
  
  If you want a brief description of the characteristics you just mentioned you can find in this article here.
  
  Directory structure:
  
  EpubFolderYouWant
     META-INF
        container.xml
     mimetype
     content.opf
     toc.ncx
  
  
  Needed files:
  
  mimetype
  container.xml
  content.opf
  toc.ncx
  
  
  And which information is contained in each file is described in the above mentioned article.