You can follow the steps below to fix the error and publish the book.
- Quit iBooks Author.
- Open the Terminal app, then enter
defaults write com.apple.iBooksAuthor SFDisableSingleFileFormat YES
- Open iBooks Author.
- Open the document that showed the Unauthorised Media error when you tried to publish it.
- Make a minor edit in the document, save the document and then close the document.
- Reopen the document, and try publish it again.