Definition:
An orphan book page is a book page where the book outline was set to <none>.
This means that there is no book containing the created book page.
Orphan pages usually occur, when you use "Create content -> book page" and forget to assign a book.

Problem:
In Drupal 5.x there was a list under "administer > content > books > orphan pages", which showed all the orphan book pages, so one could reattach the pages to a book.
In Drupal 6 this functionality was removed.

Solution:
Import the following view. It is a basic view, which shows all the orphan pages. You might want to add a display to the view.

$view = new view;
$view->name = 'Orphan_book_pages';
$view->description = '';
$view->tag = '';
$view->view_php = '';
$view->base_table = 'node';
$view->is_cacheable = FALSE;
$view->api_version = 2;
$view->disabled = FALSE; /* Edit this to true to make a default view disabled initially */
$handler = $view->new_display('default', 'Defaults', 'default');
$handler->override_option('relationships', array(
  'bid' => array(
    'label' => 'Book',
    'required' => 0,
    'id' => 'bid',
    'table' => 'book',
    'field' => 'bid',
    'relationship' => 'none',
  ),
));
$handler->override_option('fields', array(
  'title' => array(
    'id' => 'title',
    'table' => 'node',
    'field' => 'title',
  ),
));
$handler->override_option('filters', array(
  'type' => array(
    'operator' => 'in',
    'value' => array(
      'book' => 'book',
    ),
    'group' => '0',
    'exposed' => FALSE,
    'expose' => array(
      'operator' => FALSE,
      'label' => '',
    ),
    'id' => 'type',
    'table' => 'node',
    'field' => 'type',
    'relationship' => 'none',
  ),
  'depth' => array(
    'operator' => 'empty',
    'value' => array(
      'value' => '0',
      'min' => '',
      'max' => '',
    ),
    'group' => '0',
    'exposed' => FALSE,
    'expose' => array(
      'operator' => FALSE,
      'label' => '',
    ),
    'id' => 'depth',
    'table' => 'book_menu_links',
    'field' => 'depth',
    'relationship' => 'bid',
  ),
));
$handler->override_option('access', array(
  'type' => 'none',
));
$handler->override_option('items_per_page', 0);

Explanation:
The trick is to define a relationship in the view and use this relationship in the filter.
One might think it could be sufficient to filter by Book: Depth.
The problem with this is that an orphan page has no depth, since it is not part of a book and the filter only allows to filter for depth = 0 and not for isempty(NULL) .
This is accomplished with the relationship.