Hmm thinking about it, I realize that I do want books that dont specify any fonts to default to sans-serif. But it should be easily overridable, by either a user stylesheet or styles in the book itself. I guess the best way to do that would be to add an extra stylesheet at the top with
Code:
html { font-family: sans-serif }
Then if either the user stylesheet or the book stylesheet specify any fonts they will override the default.