There are two areas to custom options for Book page and Single Book are Theme option and Custom setting for Book which located in Theme option ==> Child Options such as Header, Footer, Layout, Page Title, Color, Typography
Any option is set to Default which is the current style of your Theme Option. So, if you want same settings on all pages, then you don't need to select anything. If you want a different option for Single Book page or Book page, then you can choose any other option here as your wish.
You can view the area (1) in bellow image:
This is options in Page setting