The Force.com Library Overview
The Force.com Library provides you with a complete set of platform fact sheets, white papers, books and tutorials so that you can learn about the Force.com platform and immediately start building your web applications using your free Developer Edition account. Here's a brief overview of the library you can download individually or as one zip file:
http://wiki.developerforce.com/index.php/Forcedotcom_Library_Overview
The Developer Core Resource Library is a set of resources covering all the fundamental aspects of the Force.com platform and its various components, aimed at a developer audience. These resources will get you up to speed on the platform, providing all the necessary background and pointers to get you started developing applications and solutions quickly and efficiently.
http://wiki.developerforce.com/index.php/DeveloperCoreResources
1 Getting Started
2 Release Notes
3 Reference Guides
3.1 General
3.2 Apex and Visualforce
3.3 API
3.4 Adobe Flash Builder for Force.com
3.5 Tools
4 Code and Formula Samples
5 Usage and Implementation Guides
5.1 Development on Force.com
5.2 Force.com IDE and Data Loader
5.3 Sites
5.4 Packaging and the AppExchange
5.5 Security
5.6 Mobile
6 Developer Cheat Sheets
7 Administrator Tip Sheets
7.1 General
7.2 Objects, Fields, Page Layouts, and Security
7.3 Formulas and Custom Links
7.4 Workflow, Approvals, and Email
7.5 Analytics
8 Style Guide
9 Glossary
http://wiki.developerforce.com/index.php/Documentation
Code Share is a directory of open source code projects covering all aspects of the Force.com Platform. Browse and access the source code for these projects to learn more about the code. Create an account of your own on Code Share to collaborate with project owners and other members on existing projects, and create and register projects of your own.
http://developer.force.com/codeshare