Usage of $node->title replaced by $node->label()

When accessing a node's title, do not use the property itself, but rather the Entity::label() method.

This will allow for improvements to the translatability of a node's title.

//D7
function book_outline($node) {
  drupal_set_title($node->title);
  return drupal_get_form('book_outline_form', $node);
}

//D8
function book_outline(Node $node) {
  drupal_set_title($node->label());
  return drupal_get_form('book_outline_form', $node);
}

However, setting the node title directly will still use $node->title = $new_title;.

In the same vein, entity forms exposing the label for input should use the property, not the result of the label() method. For example:

$form['title'] = array(
  '#type' => 'textfield', 
  '#title' => t('Title'),
  '#default_value' => $node->title,
);