The recently released Free Encyclopedia of Interactive Design, Usability and User Experience [interaction-design.org] has been developed under the manifesto of "Democratization of Knowledge", as it aims for people from all the far corners of the world to get free access to world-class educational materials. It also deliberately takes the opposite approach of Wikipedia or other crowd-sourcing initiatives, as all entries are written by leading figures who either invented or contributed significantly to a particular topic.
The website contains various topics that relate to data visualization. For instance, one can enjoy an elaborate introduction on visual representation by Alan Blackwell (with additional commentaries by some renowned professors like Ben Shneiderman, Clive Richards and Brad A. Myers), or more specifically dive into the topic of Data Visualization for Human Perception by Stephen Few (including a blog shortlist on which I will refrain commenting).
Be sure to check out the other topics that have been covered in a chapter, and might well be interesting to you.