Originally Posted by ibillabong
Ah, I see Adjust.
I presume that you are using iOS system fonts then? This will work even if you don't have any embedded fonts, BUT the fonts referenced must match the font name exactly as listed on iosfonts.com
You can also use a generic font family name, e.g. serif, sans-serif, cursive, fantasy, and monospace. And AFAIK, iBooks handles font fallback correctly, so you can specify multiple fonts in order of preference, ending with a generic font family name, and iBooks should do the right thing.