Neil, Earl, Peter, all, thanks for your replies. I have added your various links in the API section of the handbook: http://drupal.org/node/205296 As far as book.module is concerned, the bit that I missed was the #ahah part in the form: // Add a drop-down to select the destination book. $form['book']['bid'] = array( '#type' => 'select', '#title' => t('Book'), '#default_value' => $node->book['bid'], '#options' => $options, '#access' => (bool)$options, '#description' => t('Your page will be a part of the selected book.'), '#weight' => -5, '#attributes' => array('class' => 'book-title-select'), '#ahah' => array( 'path' => 'book/js/form', 'wrapper' => 'edit-book-plid-wrapper', 'effect' => 'slide', ), ); I found out that the #ahah attribute is completely undocumented, so I added a placeholder for it in the FAPI reference: http://api.drupal.org/api/file/developer/topics/forms_api_reference.html#aha... (my addition might take a few hours to appear). It's only a placeholder though. Someone more knowledgeable needs to complete the doc, there. If nobody does it, at least developers would be made aware that there is such a thing as #ahah in FAPI. Is that something new in Drupal 6 ? [edit: it seems to be. It is mentioned in a very unlikely place: http://drupal.org/node/114774#choice_check . Why not in Peter's AJAX page?] Blessings, Augustin.