Hi Robert,
You are confusing concepts and areas.
Fontbook was created intended for SPPB component area.
If you want to use that same font also for template areas (like Menu) you have to add extra line of Custom CSS. Here is example how it may look like:
.sp-megamenu-parent > li > a,
.sp-megamenu-parent > li > span,
.sp-megamenu-parent .sp-dropdown li.sp-menu-item > a
{font-family: 'Cinzel', sans-serif; }
Of course preview used font-family name in CSS code and use it inside my example code, and then it will work.
In the future, Helix Ultimate will get a similar tool to what SPPB has, but for now you have to use "classic" webmaster skills.