There are quite a few things on the web that you might find useful. Below are a few that I have found to be especially helpful.
A lot of people would like to see a list of does and don’ts. This is problematic, since the context and the overall design can influence whether or not two typefaces work with each other. The closest thing I’ve found to a list, however, is a matrix that appeared in U&lc magazine in September 1992 (see below). Alessandro Segalini re-discovered this a few years ago and has posted a pdf on his web site. You’ll find links to it everywhere on the web.
Matrix for matching typefaces. Appeared in the September 1992 issue of U&lc.
An article has appeared in Smashing Magazine that explains some good principles which cover the basics. It’s a good place to start.
A great article for those who have been working with type for awhile appeared in the May/June 2006 issue of Communication Arts and was written by one of the most prolific modern type designers of the last 20 years, Jonathan Hoefler. The article is substantive, thought provoking and full of sound advice. If you can’t find that issue, try the H&FJ web site, which has a page containing suggestions for mixing typefaces.
Typophile.com is a great web site for all things typographic. It has some forums/discussions that may be helpful.
A lot of the information from the blog entries was found by researching the following sites and books:
fonts.com (look under the “Learn About Type” link)
The Secret History of Letters by Simon Loxley
Meggs’ History of Graphic Design by Philip Meggs’ and Alston Purvis
The Elements of Typographic Style by Robert Bringhurst (A book as beautiful as it is useful)
This is also a page of the site where you can chime in on whatever typefaces you think work well together. Comment below.