Visual Metaphors
another interesting (if rather short and simplistic) piece from Inf@Vis! the digital magazine of InfoVis.net

where there is a link to:
The Myth of Metaphor by Alan Cooper

these provide a good companion to:
Metaphor: A Double-Edged Sword (May 2000) from the ACM digital library (which for some reason I can still access!)
or can be requested for personal use from:
http://www.syntagm.co.uk/design/articles.htm where there are lots of other interesting articles written by William Hudson