title_body: Is there a way to push epubs to iBookstore moreover using iTunes Producer?
upvoted_answer: There is most likely an API working between iTunes Producer and the iBookstore, but AFAIK Apple has not published it. It is of course much easier for Apple to control a protocol&mdash; and change if needed&mdash;when you control both client and server sides (client: iTunes Producer, server: iBookstore). This keeps development cheaper than with a published API and it might even allow Apple offload some of the work to the client side that they don't want to re-check on the server.
  
  If you are not willing to do repetitive work using iTunes Producer, I see two possible alternatives:
  
  
  checkout if any of the iBookstore aggregators (approved or others) has an API you can use. Of course a commercial service costs you, reducing earnings from the list price down from 70% when directly dealing with Apple¹. 
  programmatically drive the iTunes Producer application to do the uploading  using the Quartz Event Services². This might actually be quicker to develop than trying to interface with an API programmatically.
  
  
  ¹ I am not sure if any aggregators have an API available. Earning will  e.g. be down to 63%  using Lulu; to 60% using Smashwords
  
  ² I have done that kind of automation, programming the 'driver'
  for the .tiff to .djvu conversion in a Windows GUI application.