In order to understand all the design decisions in classes and packages distributed with TeX/LaTeX, and for my such choices of my own I've looked around for a basic but useful "Beginners Guide to Typography". Theories, thoughts etc (as well as schools of thought).
EDIT: I'm generally interested in the subject, but lean towards print, thesis and non-fiction typography.
I hope this is the right place to ask this and that there are loads of answers.
Cheers Mats