Learning Drupal 6 Module Development is a new book from Packt Publishing, written by Matt Butcher. The book is aimed at Drupal users who know PHP and want to learn module development.
I was one of the Technical Reviewers for this book, so I've had a digital copy for a while, but my final hard copy just arrived today, and I must say, it's quite cool to see my name in print! :)
This book will take you through building several different modules, linking them together through hooks, and finally, creating an install profile to incorporate them all into a distributable package. Along the way, you'll find lots of useful tips, and links to relevant topics on Drupal.org.
If you're looking to learn module development, and your focus is Drupal 6, I definitely recommend picking up a copy.
You can grab a sample chapter here (1.3M PDF).
Update: I'm giving away a free copy of this book to the person who writes the best review over at Drupal Modules.