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Drupal 6 JavaScript and jQuery by Matt Butchner
Matt Butcher has a great way of presenting his lessons.  He approaches the Drupal Framework by building on the knowledge of the Drupal code base, JavaScript, and jQuery, and  through practical examples of how to extend Drupal.  The tools you learn about, relate to a Drupal design philosophy that when understood properly, shows the programmer how to code efficiently. Drupal is a PHP open source framework that is designed through hooks to mesh well with customization.

By following this book, time and effort can be avoided by learning the right coding techniques that work with the Drupal Framework.  Understanding the Drupal hooks and coding methods are important to leverage the hard work of the Drupal developers, and avoid problems through core updates.  I would not have even posted about this book if it were not a must-have for the PHP coders using Drupal.  Definitely add this to your coding arsenal.