My colleague here at Google just published an updated set of KML documents. These documents describe the specifics of using KML to publish information and exciting content in Google Earth. The documents include:
KML Samples - This document and sample KML file represent a terrific way to learn about KML. View these to understand the basic structure of a KML file and commonly used tags.
KML 2.1 Tutorial - These hands-on instructions acquaint you with the exciting new features found in KML 2.1.
KML 2.1 Reference - This contains a description for all KML elements.