Read the available online chapters (Letter, Word and Grid) in Thinking with Type by Ellen Lupton. If you can, go to the library and read the book in print. The rules espoused in print-based typography books are often too detailed for the screen and impossible to control on the web. Still, I believe that a discipline honed over centuries in print will offer an ideal against which we can compare forms on the web.
You may find Robert Bringhurst's The Elemenets of Typographic Style, Robin Kinross' Modern Typography, and Gerrit Noordzij's The Stroke at the library. Each has profoundly influenced typography and our understanding of graphic design.