InspireBook support fontello icons in the primary navigation, user can change the icon as per there requirement. If you need extra custom fontello icons, you can follow the process given in this article.
Setup custom class
Appearance > Menus, check the checkbox for CSS Classes in Screen Options section in top right corner.
Now you will see CSS Classes textbox under your menu, here you can add or update CSS classes
Choose class for menu Icon
Followings are the list of fontello icons which have inbuilt support in InspireBook theme, use this same name as CSS class in navigation,
Important: If you have added/generated extra custom icons from fontello.com, then class name should be the name of that particular icon present on fontello.com.
Hence we are done, now save the menu settings and check in the front-end for your menu icon changes.