Lecture notes on Information Visualization by Professor Andreas Butz.
Visualization and Visual Communication by Robert Kosara.
UPD: perceptualedge.com by Stephen Few.
Color maps for Science by Kenneth Moreland. Examples, advices and more links.
Making Tufle-Style plots in R. Practical advices.
Junk Charts. Bad visualization and how to improve it.
Hive Plots: new approach to visualize networks. Don’t know if it is effective, but it is definitely interesting.
References in Russian: