Essays
Usability expert Jakob Nielsen wrote about the advantages of CSS and gave hints for how to use them already in 1997.
Even older, 1996, is the essay by Warren Steel called What's wrong with the FONT element?
A separate page collects the proposals for style sheet languages for the Web from around the time CSS was invented.
Håkon has written a PhD thesis about CSS and style sheets for the Web: Håkon Wium Lie, Cascading Style Sheets, University of Oslo, 2005.
The Wikipedia article on CSS summarizes the usage, history, advantages and limitations.