Render value of custom content type in html.html.twig
Load the node and get the field value in a preprocess hook:

function mytheme_preprocess_html(&amp;$variables) {
  if ($variables['node_type'] == 'book' &amp;&amp; ($node = \Drupal::routeMatch()-&gt;getParameter('node'))) {
    $variables['node_field1'] = $node-&gt;field_field1-&gt;value;
  }
}


This makes sense to check first for the correct content type, because then you know which fields are defined. Otherwise you would have to check if the field is present to prevent an error. But then you have already loaded the node for nothing.

Output the value in twig:

{{ node_field1 }}