Form item classes
Book element
Drupal 7 much improves the classes added to the div surrounding form items (in Drupal 6 it was hard-coded to 'form-item'). The type of form element and the form element name are now added as form-item-type and form-item-name. You can see how Drupal does this (and see the theme function you would override to change this) at api.drupal.org/theme_form_element.