Here's a commented com.apple.ibooks.display-options.xml.
Code:
<display_options>
<platform name="*"> <!-- allowed values for platform "iphone", "ipad", or "*" for all -->
<option name="specified-fonts">true</option> <!-- must be set to "true" for embedded fonts -->
</platform>
</display_options>
Can't imagine any real world case where you'd use anything but the defaults.
Perhaps there are other XMLs you'd include for other reasons.