Elements of Typographic Style for the Web
One of my favourite typography books is Robert Bringhurst's The Elements of Typographic Style. I used it a lot as a reference back when I did the occasional print design work, and I can highly recommend it to anyone interested in typography.
Originally published in 1992, Bringhurst's book does not mention anything about typography for the Web. That is understandable. But now there is a website that explains how to apply the book's working principles to the Web: Richard Rutter's Elements of Typographic Style Applied to the Web - A practical guide to web typography.
The site uses a clean, typography-oriented design that lets the reader focus on the content. For some background info and a place to leave your comments on the site, go to Richard's blog post Elements of Typographic Style Applied to the Web.
Great job, Rich!