book_blocks-8.x-1.4/config/install/book_blocks.settings.yml

block:
  settings:
    book_block_edit:
      add_sibling: 'Add sibling page'
      css: 'book_blocks_edit_normal'
      toc: true
      nav: true
      icons: true
      left_link:
        name: 'Book'
        url: '/books'
        hint: 'Show list of all books'  
      middle_link:
        name: ''
        url: ''
        hint: ''
      right_link:
        name: ''
        url: ''
        hint: ''