book_bundle.links.menu.yml

book_bundle.settings:
  title: 'Book Bundle'
  description: 'Remove or create Book and Chapter content'
  parent: system.admin_config_content
  route_name: book_bundle.settings
  weight: 10

entity.book.settings:
  title: 'Book'
  description: 'Configure a Book entity type'
  route_name:  entity.book.settings
  parent: entity.node_type.collection
entity.book.collection:
  title: 'Books'
  description: 'List of books'
  route_name: entity.book.collection
  parent: system.admin_content
entity.book.add:
  title: 'Add Book'
  description: 'Add Book'
  route_name:  entity.book.add_form
  parent: entity.book.collection

entity.chapter.settings:
  title: 'Chapter'
  description: 'Configure a Chapter entity type'
  route_name:  entity.chapter.settings
  parent: entity.node_type.collection
entity.chapter.collection:
  title: 'Chapters'
  description: 'List of chapters'
  route_name: entity.chapter.collection
  parent: system.admin_content
entity.chapter.add:
  title: 'Add Chapter'
  description: 'Add Chapter'
  route_name:  entity.chapter.add_form
  parent: entity.chapter.collection