If you set a font for a language or a module in the book installer, there is no way to unset/reset it from the interface. The only way I know of is to go to the BookFont.properties file and delete the entry manually.

It would be handy to have an "Unset" button either in the primary interface (adjacent to the Font button) which would be enabled only if a font was set for this module / language. Alternatively, a "Default" button in the Font dialog would also do the trick. I personally prefer the former option, unfortunately it would increase the size of the book installer a bit.