At the bottom of the book (in the book editor), always show menu display options. This option includes: (Menu style (of chapters): View [Auto] [Classic] [Tree])
Default is [Auto]: The program will automatically calculate. If the book chapter contains many subchapters (more than 15), the display method is [Tree], otherwise the display method is [Classic].
[Classic]: Display all chapters in the book, no need for [+],[-] buttons.
[Tree]: Only display level 1 (main chapters), use the [+] button to display the internal parts or the [-] button to collapse (hide the internal parts).