Source SDK: Mastering the Source/SDK Reference Docs
The Source SDK Reference Documents is a section located under the Documentation panel in the Application Browser. It is a link to the main directory page of the Valve Developer Community wiki, which houses complete tutorials on all aspects of the Source SDK and it's functions, and is the choice tutorial site sponsored by Valve Corporation itself.
This is the link to the Developer Community Directory, clicking the SDK Reference Docs tab in the Application Browser will take you directly to this same link.