Create book node via drupal_execute

I'm trying to create book nodes using drupal_execute (I'm importing a bunch of nodes from content stored in files). I've been trying everything I can think of to make this work and I'm really frustrated. I can create the nodes, but I can't make one page a child of another page. Instead nodes which should be parent-child are siblings under the top-level book node.

Here's a chunk of the code I'm using:

    // values like $plid (nid of the parent page) and $bid (nid of the top-level book) have been set earlier in the code.

    // Create a node based on the parsed content                                
    $form_state = array();
    module_load_include('inc', 'node', 'node.pages');
    $node = array('type' => 'book');
    $form_state['values']['title'] = $title;
    $form_state['values']['body'] = $body;
    $form_state['values']['name'] = 'superadmin';
    $form_state['values']['op'] = t('Save');
    $form_state['values']['book'] =
      array('bid' => $bid,
            'plid' => $plid,
            'weight' => $weight,
            );
    drupal_execute('book_node_form', $form_state, (object)$node);

I saw a somewhat related post: http://drupal.org/node/252341, but the advice there did not help. I've tried the following code and it also does not work.

    // Create a node based on the parsed content                                
    $form_state = array();
    module_load_include('inc', 'node', 'node.pages');
    $node = array('type' => 'book');
    $form_state['values']['title'] = $title;
    $form_state['values']['body'] = $body;
    $form_state['values']['name'] = 'superadmin';
    $form_state['values']['op'] = t('Save');
    $form_state['values']['book'] =
      array('bid' => array('value' => $bid),
            'plid' => array('value' => $plid),
            'weight' => array('value' => $weight),
            'module' => array('value' => 'book'),
            );
    drupal_execute('book_node_form', $form_state, (object)$node);