"iBooks also has a very simple linking architecture that enables you to link directly to books within the Store.
This could be used for marketing from your own website or online advertisements, or it could be used within
an EPUB to direct a customer to other related titles at the end of a book." - iBooks Asset Guide 5.1 Rev 1
I remember reading in one of apple's guides, can't seem to find it now, that you can't link to companies that compete directly with Apple (ie. Amazon.com)
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