The SDK documentation consists of two main parts, the additional content and the Framework Reference section.
In the additional content section you can read introductions to the major areas of the EPiServer platform as well as view class diagrams on certain architectural building blocks such as workflows and virtual path providers.

EPiServer CMS 5 R2 Documentation

The SDK documentation found here is compatible with the EPiServer CMS 5 R2 release.

Online Resources

Find out where to get more information on different topics.

Getting Started

This section is a good place to start for the new EPiServer developer.

Developer's Guide

Apart from the framework reference section this is the main part of the SDK. This is a good place to start when trying out new areas of the EPiServer framework such as workflows, data sources, theming and the new and enhanced property types introduced in EPiServer CMS.
In the How To-section of this block you will find short examples on how to create EPiServer specific code.

Help on Help (HTML HelpViewer)

If you want to find out how to improve your effectivity while working with the Help Viewer, this section should contain all the information you need.