Use the URI, or ISBN if available.
An unambiguous reference to the resource within a given context.
Recommended best practice is to identify the resource by means of a
string conforming to a formal identification system.
Used for the dc:identifier metadata parameter in the OPF file, as well
as dtb:uid in the NCX file.
Identifier type should only be:
EPub::IDENTIFIER_URI
EPub::IDENTIFIER_ISBN
EPub::IDENTIFIER_UUID