Listing book 'sibling' pages?

Thanks a lot merlinofchaos for your reply: it gave me a great idea on how to proceed. I tried using the 'Node from URL' default argument option, but it didn't work as:

1) the query in live preview showed me it'd use the node from url as 'parent', while I'd need it to use the node's book parent as parent:

WHERE (node.status <> 0) AND (node_book_parent.nid = 124)

(where 124 is the node I need siblings of, and as such it doesn't work as parent for this purpose)

2) I heavily use aliases, so the node id would not be in the url anyhow.

However if I manage to get the node's book parent via php somehow, trying it out as argument in the live preview shows me it'd indeed get me the results I need, so thanks again.

moofie & merlinofchaos: thank you! Using your suggestions I was able to create the view I needed. Here it is, in case somebody else needs something like it:

$view = new view;
$view->name = 'list_sibling_book_pages';
$view->description = 'List sibling book pages';
$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(
  'nid' => array(
    'label' => 'Book parent',
    'required' => 1,
    'id' => 'nid',
    'table' => 'book_parent',
    'field' => 'nid',
    'relationship' => 'none',
  ),
));
$handler->override_option('fields', array(
  'title' => array(
    'label' => '',
    'link_to_node' => 1,
    'exclude' => 0,
    'id' => 'title',
    'table' => 'node',
    'field' => 'title',
    'relationship' => 'none',
  ),
));
$handler->override_option('sorts', array(
  'p' => array(
    'order' => 'ASC',
    'id' => 'p',
    'table' => 'book_menu_links',
    'field' => 'p',
    'relationship' => 'none',
  ),
  'weight' => array(
    'order' => 'ASC',
    'id' => 'weight',
    'table' => 'book_menu_links',
    'field' => 'weight',
    'relationship' => 'none',
  ),
));
$handler->override_option('arguments', array(
  'nid' => array(
    'default_action' => 'default',
    'style_plugin' => 'default_summary',
    'style_options' => array(),
    'wildcard' => 'all',
    'wildcard_substitution' => 'All',
    'title' => '',
    'default_argument_type' => 'node',
    'default_argument' => '',
    'validate_type' => 'php',
    'validate_fail' => 'not found',
    'break_phrase' => 0,
    'not' => 0,
    'id' => 'nid',
    'table' => 'node',
    'field' => 'nid',
    'relationship' => 'nid',
    'default_options_div_prefix' => '',
    'default_argument_fixed' => '',
    'default_argument_php' => '',
    'validate_argument_node_access' => 0,
    'validate_argument_nid_type' => 'nid',
    'validate_argument_php' => '$node = node_load($argument);
        if(isset($node->book)) { // book page
              $parent = book_link_load($node->book[\'plid\']);
              $handler->argument = $parent[\'nid\'];
              return TRUE;
        }',
  ),
));
$handler->override_option('filters', array(
  'status' => array(
    'operator' => '=',
    'value' => 1,
    'group' => '0',
    'exposed' => FALSE,
    'expose' => array(
      'operator' => FALSE,
      'label' => '',
    ),
    'id' => 'status',
    'table' => 'node',
    'field' => 'status',
    'relationship' => 'none',
  ),
));
$handler->override_option('access', array(
  'type' => 'none',
));
$handler->override_option('items_per_page', 100);
$handler->override_option('style_plugin', 'list');
$handler->override_option('style_options', array(
  'grouping' => '',
  'type' => 'ul',
));
$handler = $view->new_display('block', 'Block', 'block_1');
$handler->override_option('block_description', 'List sibling book pages');
$handler->override_option('block_caching', -1);