Since the EPUB format is based on HTML, links can be used to augment and enhance book content. Links can be within a document, as in the case of an index or endnotes, or can link to the web or other external resources such as an author or publisher website.
Apple Books also has a very simple linking architecture that enables you to link directly to books within Apple Books. 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. For information about linking to the store, see Link to Apple Books in the Apple Books Publisher User Guide.