HOW TO WORK WITH FONTS
The font can be changed either for the whole book (global font assigment) or for the selected text fragment only (local font assignment).  Being in the Reader mode you can change the global font only,  while being in the Editor mode you can change both global font and local font as well. 
Changing fonts from the Editor mode.  If nothing is selected in the main Book Designer window, the font is changed for the whole book. Otherwise the font is changed for the selected text fragment. 
 
To change the font. Click on a desired font name in the left window of Book Designer "Change Font.
To change the font size. Click on a desired font size in the right window of Book  Designer "Change Font".
add: add new font to the "font name" list 
remove: remove selected font from the "font name" list 
default: restore default the "font name" list 
undo: reject last changes
undo all: return to the global font settings which took place before the opening of Book Designer "Change Font" window.
 
Comment. You can change the font settings as many times as you want without closing the "Change Font" window.