Additional Information

Clean Up Your Mess - A guide to visual design for everyone
Usability Guidelines:
- Principles for Usable Design
- Principles of Accessible and Universal Design
More than Just Eye Candy by Clark & Lyons. This article is good for learning about the types of visuals used in instruction; however, the example graphics they use are dated as the article was written in 2003.
Typography and readability by Itkonen (Translated from Itkonen, M. 2007. Typofrafian käsikirja. 3. edition. Helsinki: RPS-yhtiöt.)
Color Theory for the Color Bind is a nice walk-through of things to consider and strategies to use to make your visuals and text color-blind friendly

References

Brumberger, E. (2004). The rhetoric of typography. Technical Communication, 51(1), 13-24.
Fogg, B. J., Soohoo, C., Danielson, D. R., Marable, L., Stanford, J., & Tauber, E. R. (2003). How do users evaluate the credibility of web sites?: A study with over 2,500 participants. In Proceedings of the 2003 Conference on Designing for User Experiences (pp. 1-15). ACM.
French, M. M. J., Blood, A., Bright, N. D., Futak, D., Grohmann, M. J., Hasthorpe, A., ... & Tabor, J. (2013). Changing fonts in education: how the benefits vary with ability and dyslexia. The Journal of Educational Research,106(4), 301-304.
Haugen, J. L. (2015) Becoming a more thoughtful user experience designer. UX Magazine
Mehlenbacher, B. (2002).Assessing the usability of on‐line instructional materials (pdf, 60k). New Directions for Teaching and Learning, 2002(91), 91-98.
Metzger, M. J., Flanagin, A. J., & Medders, R. B. (2010). Social and heuristic approaches to credibility evaluation online. Journal of Communication, 60(3), 413-439.
Rello, L., & Baeza-Yates, R. (2013, October). Good fonts for dyslexia. In the Proceedings of the 15th International ACM SIGACCESS Conference on Computers and Accessibility (p. 14). ACM.
Robins, D., & Holmes, J. (2008). Aesthetics and credibility in web site design (pdf, 752k). Information Processing and Management, 44(1), 386-399.
Song, H., & Schwarz, N. (2008). If it's hard to read, it's hard to do: Processing fluency affects effort prediction and motivation. Psychological Science,19(10), 986-988.