Hey Dan, I would definitely recommend reading these sources for getting a comprehensive understanding about general design principals and core ideas:

The Gestalt Principles The guiding theory in understanding how the human brain process images and visual data. http://graphicdesign.spokanefalls.edu/tutorials/process/gest...

Web Typography Great resource for understanding typography and especially for the web http://webtypography.net/

A List Apart I am sure you are already familiar with this awesome publication by the Happy Cog crew and other contributors. To me this covers most of the on screen design issues in today's world and it keeps on evolving which is awesome. http://www.alistapart.com/

I Hope this help!