Andy Kirk has recently updated his collection of resources on his Visualizing Data blog. This is a comprehensive collection that contains nearly 300 tools. It's well-organized into a variety of tagged categories such as Data Handling, Charting, Programming, Multivariate, and Color among others. This is definitely worth bookmarking.
William Zinsser authored a classic writing book called, On Writing Well, which identifies seven key principles for effective writing. As Ben Jones shows in this article, those same principles apply directly to visual communication.