Five Simple Steps to Better Typography: Hierarchy—Weight
http://www.markboulton.co.uk/journal/comments/five_simple_steps_to_better_typography_part_5/

Boulton, Mark
Mark Boulton
2005

Abstract:

Typeface weight, and the choice of weight, is perhaps one area of typography that to most designers is simply a matter of choice. That choice is dictated by answering a design problem which is aesthetically, or content, motivated. What many designers do not realise is that there are rules which should govern the choice of weight - a typographic pecking order - which when followed, aids the designer's typesetting and can produce stunning results.