The Theming Handbook is being reorganized!
While many people say that theming is one of the better organized topics in Drupal's documentation, many have expressed frustration that relevant pages are located in a variety of different branches, and even separate handbooks. The documentation team has also decided that this has created an unsustainable path for forward maintenance. Therefore, as a first step, we are going to merge nearly all theming-related information into a single Consolidated Theming Guide on Sunday, 23 November, to be located at http://drupal.org/theme-guide (currently pointing to the Drupal 6 Theme Guide). Existing links, both to individual nodes numbers and path aliases, will be preserved as much as possible.
A theming documentation workgroup is now forming to focus on reorganizing the new book's navigation hierarchy. For further information on the project and how to join in, please visit the group's project page.
Note that as part of the current overall drupal.org redesign project, there are more general discussions going on about a full re-structuring of Drupal's documentation as well, which all are encouraged to take part in.