I've been collecting resources on my own over the past year or so as I've taken a deep dive into Drupal. I'm now in the process of training some of my staff and lead contacts in Drupal. In several cases, we'll need to buy some duplicate copies of books and thought this might be a nice place to provide a list and some of my thoughts on their usefulness in my work.
Books
My favorite book for learning Drupal has been Front End Drupal: Designing, Theming, Scripting by Emma Jane Hogbin and Konstantin Kafer. This is a book I've returned to from time to time. In particular, I like chapter 5 of this one. It breaks down the approach on controlling content in the various template files; node, content type, page, etc. An easy read that illustrates an overview of what is possible in D6. If I was on a limited budget and needed to learn as quickly as possible, I'd start here.
Pro Drupal Development by John K VanDyk is the reference book on Drupal. I started with this book. This is the reference book that is one you'll keep going back to. It's not for the novice, but provides the detailed view of the API's and other components within the framework. In hindsight, I'd start with Front End Drupal if I were just starting. This is focused at a developer who is coming to Drupal with a depth of experience in web development.
Drupal Multimedia by Aaron Winborn. This book pulls together some of the more in-depth modules and approaches necessary for developing sites that will rely upon different types of media. This one is for the novice who wants to add some sophistication to their site. It's a great overview on how to do an image gallery and tie in jQuery UI components.
Building Powerful and Robust Websites with Drupal 6 by David Mercer. This is a nice overview book for novices. I've gone back and re-read a few chapters. I'd go with this and Drupal Front End if I were just starting out. Mercer spends enough time on the basics that you can follow along on building a project via the book.
Drupal 6 Themes by Ric Shreves. This is a nice overview on the theming approach in Drupal. If you don't know what override and intercept mean, this is a good place to start. Override and intercept are key components to understanding how to bend Drupal to your will.
Learning jQuery by Jonathan Chaffer, Karl Swedberg. Good overview on the basics of jQuery. Nice reference material with several good examples.
jQuery: Novice to Ninja by Earle Castledine and Craig Sharkie. My latest favorite on jQuery. Lot's of good examples and good overview to how it
eBooks
Theming Drupal: A first timer's guide by Emma Jane Hogbin. This workbook style book is a fantastic supplemental for the Front End Drupal book (mentioned above). The book is skills based and is intended to step through the process of designing a theme from scratch. Scratch might be using a base theme such as Zen or 960 grids or via a Photoshop file.
The reader is taken through descriptions of the various components that make up a theme in Drupal. Beginning with the elements that make up a page (regions, blocks, HTML) through descriptions of the interaction with Drupal nodes, pages and variables. Emma Jane spends time describing how Drupal sets page variables and how those variables are displayed in the page via the template.
In the section Displaying Content with More Precision, is reference material you'll come back to time and again; how to pluck out the labels and field values from node variables. Flatter portions of my forehead could have been saved by bookmarking this type of reference material. My staff should take this into account when I hand out their next assignments... :) You get extra points for not making the same mistakes I've made.
PHP Web 2.0 Mashup Projects by Shu-Wai Chow. This book has one chapter in it that made me buy it. The author has a screen scraping tutorial using the California Highway Patrol site. I've been working on integrating this to a Managing News instance to begin to collect traffic information for our regional transportation issues. This is still a work on paper at this point. I'll need to find method to do pull in some PHP functions (maybe via a custom module) and turn that into a feature.
Video/DVD
Understanding Drupal - Lullabot
Theming Basics for Drupal - Lullabot
Advanced Theming for Drupal - Lullabot
Learning Views - Lullabot
The CSS Video Crashcourse - Sitepoint
Screencasts Sites
Some of my favorite screen cast sites include
Mustardseed
Drupal Therapy
Got Drupal