I am currently reading not just any other Drupal book. With its steady flow of publication goodness, Packt Publishing has always been, in my opinion, the perfect B+ student: hardly ever outstanding. But its latest Drupal title, Learning Drupal 6 Module Development, is exceptional. The book, so far, has very much surpassed my expectations. And what an outstanding writer Matt Butcher is!
I love the way he explains the module hook mechanism in chapter 1. Indulge me by imagining modules as women, and Drupal as a man with a mask.
"At predefined moments in this step-by-step  process, Drupal
executes hooks. What does this mean? In short, it means that Drupal examines some or all of the currently enabledmodules, looking for functions that follow specific, predefined patterns. Some have linked this process to the 'callback' method often used in event handling models. It is similar to this, but  more dynamic." (p.11)
"Drupal then continues its step-by-step processing of the request, perhaps executing many other hooks as it goes. Once all the steps have been completed and a response sent to the user, Drupal cleans itself up and exits." p.11.
I will come up with a serious review in about a week from now. But so far, I am jubilating. Grab your own copy. It is as good as Pro Drupal Development, but different.