Perceptual Edge has a library of excellent articles written by the Stephen Few, author of the site. His examples were also informative; each example analyzes and redesigns a statistical graphic. The discussion groups have some thoughtful ideas, but are not as useful as Tufte’s Ask E.T. forum. (By the way, I wish the website was as good as the content. Something as simple as the navigation links worked only intermittently.)
Few has two books that look interesting: Information Dashboard Design and Show Me the Numbers: Designing Tables and Graphs to Enlighten.