Using the Calibre's Editor, I just open this new file: com.apple.ibooks.display-options.xml
This file contains:
Code:
<?xml version="1.0" encoding="UTF-8"?>
<display_options>
<platform name="*">
<option name="specified-fonts">true</option>
</platform>
</display_options>
And that's it, my fonts (and CSS) work in ibooks.