How to Export Your iBooks Author e-Book through iTunes Producer
After you finish creating your e-book in iBooks Author, you need to export it to prepare it for distribution through the iBookstore or elsewhere. If you don’t charge for the e-book, you can both make it available for free in the iBookstore and distribute it for free outside the iBookstore. But if you charge for the e-book, you must distribute it only via the iBookstore.
When your e-book is ready to be exported, follow these steps:
Start the export of the book in iBooks Author as an .itmsp file by clicking the Publish button in the toolbar.
The settings sheet opens.
Enter a filename in the Save As field.
Specify a location where you want to save the file in the navigation section of the settings sheet.
Click Publish to complete the export.
The iBookstore has a maximum file size of 2GB for e-books, and Apple recommends keeping e-books less than 1GB in size. It also notes that books larger than 20MB can’t be downloaded over cellular (3G and 4G) connections, just over Wi-Fi or via iTunes.