I strongly believe that in order to do good interaction work you really just need to understand typography well. That might not be a popular viewpoint around here, but I encourage you to consider some of these references.

I recommend Josef Muller-Brockmann's Grid Systems in Graphic Design: http://www.amazon.com/Grid-Systems-Graphic-Design-Communicat... or Emil Ruder's Typographie: http://www.amazon.com/Typographie-Manual-Design-Emil-Ruder/d....

There are many, many other good resources but those are important primary sources. So is The Elements of Typographic Style, by Robert Bringhurst: http://www.amazon.com/gp/product/0881792128/ref=pd_lpo_sbs_d....

It's important not to oversell the touchy-feely/what-looks-good side of this. Font design is certainly about planning and calculation, it's just that those calculations need to be based on optics rather than geometry.

Instead of perfect circles, we use a shape that is taller than it is wide; instead of splitting the space in half, cross-strokes should be higher than the middle; different strokes need to be different widths in order to appear the same, etc.

There are plenty of actual scientific studies and theory behind this. See Ruder's Typography [0] and Hochuli's Detail in typography [1] for more info.

[0] http://www.amazon.com/Typographie-Manual-Design-Emil-Ruder/d...

[1] http://www.amazon.com/Detail-Typography-Jost-Hochuli/dp/0907...