I've dealt with many, many different types of software over the years, and rarely have I come across an application or platform with better documentation than Drupal. It's truly amazing that this documentation was created almost entirely by volunteers, and it's a great example of the dedication to improving the system and it's usability. I'll try and update this post as I find particularly helpful Drupal Documentation sites.
Here are the most useful and important:
The Drupal Handbook. I usually tend to just zip through tech manuals, and search for items as I need them, but the Drupal Handbook is truly a must read for developers.
Pro Drupal Development, by Matt Westgate and John VanDyk is the first book dealing specifically with Drupal Development, and though it was a hard read at first, largely do to my relatively small knowledge of PHP (which I improved on a lot with the O'Rielly PHP/MySQL book), I picked up a lot of great knowledge from the book, and now feel a lot more comfortable theming. The biggest thing I learned from the book has to do with the fact that Drupal's functionality stems from a series of "hooks," many of which can be overwritten by your theme's template.php file (if you're using a phptemplate theme, which is highly recommended). The idea of hooks is really confusing at first to someone who is fairly new to programming languages, but I finally feel like I've wrapped by head around them, and now can try to dive a little deeper into developing awesome sites that don't look like the standard Drupal site (as this site does at the moment).
Drupal Video Casts. But... I still prefer to be shown how tech works (don't we all?), and so these VideoCasts are invaluable for me. My man Josh Koenig, who started the AWESOME Drupal Dojo, has a TON of videos. Josh who was one of the main people to help usher me into online politics, he "evangelized" me to Drupal (or is that converted?), and he's one of my co-bloggers over at Future Majority. He's one amazing teacher, and a scholar to boot, so I highly recommend checking out his stuff. I'm going to create a separate post on video resources, that I will then link to via a menu item.
Drupal API Reference. Once you start getting a little deeper into Drupal, probably about the time you try to figure out what the hell theming is and how to do it, as I am doing right now, this api reference guide will be your best friend. This guide is part of what's driving me to learn more every day: knowledge is intimidating when you don't posses it, and there's only one way to get over it, and that's to learn. I guess I'm really a geek, because I'm really loving it!