Thanks.
Have been making epubs for iBooks for a year and just found out it wasn't using my embedded fonts...
I was a little surprised that I had to supply the XML myself. Since it's just 169 bytes and never changes (theoretically, you could use it to disallow specified fonts, but why would anyone do that?) I thought it would be included and generated on the fly.
If you ever revisit this, I suggest that option, can allow user supplied as well if you think useful.
Also, a few lines to explain just what the file does and why it's needed might be useful.
I found this:
http://guidohenkel.com/2015/04/custom-fonts-in-ibooks/