The book manager. Type:

public BookManager::bookTreeOutput(array $tree)

public BookManager::getActiveTrailIds($bid, $link)

The book being displayed. Type:

public BookRemoveForm::__construct(BookManagerInterface $boo

protected BookManager::findChildrenRelativeDepth(array $book_link)

protected BookExport::bookNodeExport(NodeInterface $node, $children = '')

Books Array. Type:

protected BookManager::addParentSelectFormElements(array $book_link)

public BookManagerInterface::getLinkDefaults($nid)