[Sirtel]

Probably Times New Roman is already embedded in the book. The easiest way to get rid of it is to edit the css. You can also try converting and checking the box Fonts in the Look & Feel -> Styling -> Filter style information. Note that after that, you must convert again to embed Avenir.

If that doesn't help, then Times New Roman is specified in some other way, probably in the inline styles. Editing the html is again the simplest way of getting rid of it.