The best book I know on the topic is Robert Bringhurst's The Elements of Typographic Style. Highly, highly recommended.
Other great books: Ellen Lupton's Thinking With Type and James Felici's The Complete Manual of Typography. Both are really solid.
Books that would be worth checking out at a library: Richard Hendel's On Book Design and Ruari McLean's Typographers on Type. More philosophical in nature, less practical.
It's a classic typography book translated to a web context with the relevant CSS styles. Pretty cool.
For a more easily digestible (and interactive!) overview, the one that comes to mind is the Interactive Guide to Blog Typography: http://www.kaikkonendesign.fi/typography/
If you'd like to check out more resources, hackdesign, a newsletter of curated design-related links, is focusing on typography this month: http://hackdesign.org/courses/
This online version of "Designing for the Web" [1] is a very good (and free!) read. Definitely helped me understand some of the principles of typography, even layout and colour. The "Getting Started" and "Research" sections can be skipped without issue.
Other great books: Ellen Lupton's Thinking With Type and James Felici's The Complete Manual of Typography. Both are really solid.
Books that would be worth checking out at a library: Richard Hendel's On Book Design and Ruari McLean's Typographers on Type. More philosophical in nature, less practical.