How to include book traversal links on a custom twig template page

Question

1. I use the Book module to create books with chapters.
2. When I use the default twig template node.html.twig to display a book chapter, a section titled "Book traversal links" is included, with links to the previous chapter, the next chapter, and to the book. The debugger says these links come from the book-navigation.html.twig template.
3. Instead of the default template, I'm using a custom twig template node--chapter.html.twig to display chapters.

QUESTION:

How can I get the same book traversal links onto my custom page?

NOTES:

1. I checked for a block containing the book traversal links that I could just insert in my twig template, but I don't see one.
2. I have checked the $variables array in the twig template to see if maybe a list of the chapter's siblings is already there, but don't see one.
3. I'm already using HOOK_preprocess_node(&$variables) to find the book parent book's id, so I can use it to find the chapter's siblings, if that's the way to go.

Answer 1

Alas, there is no block for this; the book navigation links are hacked on to the node view in core.

From core/modules/book/book.module:

/**
 * Implements hook_ENTITY_TYPE_view() for node entities.
 */
function book_node_view(array &$build, EntityInterface $node, EntityViewDisplayInterface $display, $view_mode) {
  if ($view_mode == 'full') {
    if (!empty($node->book['bid']) && empty($node->in_preview)) {
      $book_node = Node::load($node->book['bid']);
      if (!$book_node->access()) {
        return;
      }
      $build['book_navigation'] = [
        '#theme' => 'book_navigation',
        '#book_link' => $node->book,
        '#weight' => 100,
        // The book navigation is a listing of Node entities, so associate its
        // list cache tag for correct invalidation.
        '#cache' => [
          'tags' => $node->getEntityType()->getListCacheTags(),
        ],
      ];
    }
  }
}

The good news is that this should work with custom templates as long as you are using the full view mode.

I have a custom node template and I still get the navigation links on my books at the bottom of the content. (Unfortunately, whether I want them there or not-- as per the above code, they are forcibly added and there is no UI to control them.)

Answer 2

You can code the book navigation into your own block.

In this case, you will need to override book_node_view() so that the nav links are not displayed twice.

This may give you an idea (not complete code, cannot be used by itself; inspired by this post):

class BookNavigationBlock extends BlockBase {

  /**
   * {@inheritdoc}
   */
  public function build() {
    $build = [];
    $node = \Drupal::routeMatch()->getParameter('node');
    if (isset($node)) {
      if ($node->bundle() == 'book') {
        $build['book_navigation'] = [
          '#theme' => 'book_navigation',
          '#book_link' => $node->book,
          '#weight' => 100,
          // The book navigation is a listing of Node entities, so associate its
          // list cache tag for correct invalidation.
          '#cache' => [
            'tags' => $node->getEntityType()->getListCacheTags(),
          ],
        ];
      }
    }
    return $build;
  }