Actually, he addresses the topic in detail, starting with not letting your developers draw icons that all end up looking like squashed bugs. He spends time on how to come up with icons to describe etherial concepts and how to avoid the dependance on icons to identify functionality.
Icons should be like short-cuts. They are an easily identifiable trigger for a piece of functionality, assuming that you know what it means. There should be text along with the icon to create that mapping in the users brain.

-pete
Entropy is not what is used to be.