As we're trying to do a better job providing some great Wrox content for PHP developers, you're going to start seeing the results of an intense PHP focus over the next few months with several new Wrox books and Blox coming. One of the first is Victor Kane's Leveraging Drupal
and from that, we've published a section as a stand-alone Wrox Blox - Upgrading Your Online Website Application to Drupal 6.x.
I'm actually anxious to have time to read this myself, as Victor was very helpful to me some months back when we made the decision to go with the PHP-based VBulletin for these new forums and with that, to add some wiki and blog features (still to come with Drupal). There are other Wiley sites using Drupal too.
Anyway, the beginning of the description for Victor's Blox
This Wrox Blox shows you how to perform a complete upgrade from Drupal 5 to Drupal 6 for a simple but non-trivial Drupal site involving the Drupal core itself, and several indispensable modules. The author gives step-by-step instructions for carrying out this upgrade, outlining the best practices to be followed. These include the recommendation of using a test site in conjunction with a subversion repository for version control and deployment to production.