The The Grammar of Graphics (Statistics and Computing) by Leland Wilkinson is also great, though more focused on building graphing systems. I believe it is the inspiration for a lot of d3.js
http://www.amazon.com/The-Grammar-Graphics-Statistics-Comput...
That Stephen Few book is pretty much all you need, but you can also check out Edward Tufte's books, and Juice Analytics' white papers (http://www.juiceanalytics.com/).
http://www.perceptualedge.com/articles/Whitepapers/Dashboard... http://www.perceptualedge.com/articles/Whitepapers/Common_Pi...
The author also has written a great book on dashboard design.
Operations dashboards are essential for any (tech?) company.
Business dashboards are another beast all together.