There are three main sources of information that you can draw on to learn more about how to use Open Inventor:
Toolchest
menu that should be
showing on your desktop when logged into an Athena SGI. The
Help
menu option will lead you to the collection of
online books. This book is also printed in a softcover format by
Addison Wesley.
man
(online manual) pages for these classes, with the
same name but usually beginning with the letters "So". For example,
to find out more about Cone
nodes, you could just use the
command "man SoCone"
from the Athena prompt.