News flash: I'm going to Australia in February, so this Web site may be switched off at any time.
The best way to learn the Icon programming language is to read the inventor's book:
Griswold, Ralph E., and Madge T. Griswold. The Icon programming language. 2nd ed. Prentice-Hall, 1990. ISBN 0-13-447889-4.
The Icon Project at the University of Arizona maintains a homepage for the Icon programming language. This includes a Frequently Answered Questions (FAQ), an online reference manual, the definitive ftp archive, and many other useful Icon resources, including an object-oriented language derived from Icon that is called Idol.
David Hanson at Princeton has written an excellent technical summary, A brief introduction to Icon.
john@nmt.edu