I finally got a copy of Information Visualization by Colin Ware and started reading it. Information Visualization and Readings in Information Visualization edited by Stuart Card, Jock Mackinlay, and Ben Schneiderman are considered the classics in the field of Information Visualization (or InfoVis).
In Information Visualization, Ware evaluates visualizations using the scientific methods of psychology. Perception, vision, color, etc. are discussed and used to reach conclusions for creating visualizations.
The section on various flow visualization techniques was very useful since I’ve been thinking about line-integral convolutions (LIC) lately. There were also discussions of choosing colors, using texture maps, 3D shading, and a few plot types that have changed the way I will do some types of visualizations in the future.
In summary, I would say this is a useful book if you are serious about improving your visualizations. (Colin Ware has a new book coming out this April that looks interesting as well.)