Is it possible to make the Book Outline a mandatory field when creating a new book page?
EDIT: On review, I see this question is D7 specific. There may be subtle things wrong with my code snippet. It should be fundamentally the same.

If something should never happen, I prefer to make it never-an-option.

If you want to make all book pages default to being new books:

/**
 * Implementation of hook_form_FORM_ID_alter() for node_form.
 */
function mymodule_form_node_form_alter(&amp;$form, &amp;$form_state) {
  if (book_type_is_allowed($form['#node']-&gt;type)) {
    // Remove the "none" option.
    unset($form['book']['bid']['#options'][0]);
  }
}


Once that's done, you can leave it be or set $form['book']['bid']['#default_value'] explicitly to be a new book, or a specified book bid of your choice. It will probably naturally fall into creating new books, as that is the first option in the selector after you remove none.