I am wondering if there is an API that will allow me to push epubs, with all the needed meta data, to Apple iTunes/iBookstore. Currently I am using the desktop app iTunes Producer.
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— and change if needed—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.
² I have done that kind of automation, programming the 'driver'
.djvu conversion in a Windows GUI application.