I’ve included the font’s License.txt in the Misc folder (following Sigil’s convention; if you’re using another tool you might keep it in the Fonts directory). Excerpt from content.opf:
Code:
<manifest>
…
<item id="RM-Ginger.ttf" href="Fonts/RM-Ginger.ttf" media-type="application/vnd.ms-opentype"/>
<item id="RM-Ginger-License.txt" href="Misc/RM-Ginger-License.txt" media-type="text/plain"/>
<item id="RM-Ginger-Readme.txt" href="Misc/RM-Ginger-Readme.txt" media-type="text/plain"/>
…
</manifest>
The Readme and License files are not in the <spine> at all, and are not linked to in the book, but if someone unzips the EPUB file to get at the font, my responsibilities under the redistribution license are satisfied.
If you want to (or if the font’s license requires you to) do more, you might include a colophon somewhere, perhaps something like—
Chapter headings have been set with Super Cool Font, <url here>, © 2010 Ty Pographer. For license terms click here.
—with “here” being a link to that information, formatted as you will.
(In my particular case, I’ve included similar language but without linking to the license terms.)