Straight from Mountain View, courtesy of Googler John Gardiner’s Using Google Earth blog, news that the KML reference materials have been updated. Check out:

• A KML 2.1 tutorial, including explanations of regions, textured 3D models, incremental updates and more.
• A KML 2.1 Reference, containing a thorough run-through of tags and attributes.
• KML Samples which are self explanatory. This is also where you find the proper KML MIME types you need to add to your server’s config files to get them to serve KML properly.
• And finally, a proper link to the KML 2.1 XSD schema.