Developers Guide:
http://code.google.com/apis/kml/documentation/topicsinkml.html
Interactive KML xml data on map (very useful to understand KML):
http://kml-samples.googlecode.com/svn/trunk/interactive/index.html
Other intesting KML xml data files:
KML, PHP, MySQL
http://code.google.com/apis/kml/articles/phpmysqlkml.html
KML Reference, defines XML structure
http://code.google.com/apis/kml/documentation/kmlreference.html
http://faculty.unlv.edu/jensen/html/Google/Maps/index.htm
Geo icons
http://econym.org.uk/gmap/geicons.htm
Free icons
http://www.famfamfam.com/lab/icons