This topic comes up from time to time...

An article from Smashing explaining the basics)

tl;dr?

here's the gist of the article, as gleaned form the article headers & the conclusion:

The fact that there are no hard and fast rules about combining typefaces can make the process of making good choices time-consuming and maybe even a little exhausting. But it’s also nice to have a handy set of principles, as well as an understanding of certain typographic situations to avoid, to guide the process as quickly as possible to a pleasant typographic result.

Combine a Sans Serif with a Serif Avoid Similar Classifications Assign Distinct Roles Contrast Font Weights Create a Variety of Typographic Colors Don’t Mix Moods Contrast Distinct with Neutral Avoid Combinations That are Too Disparate Keep It Simple — Try Just Two Typefaces Use Different Point Sizes

Reminds me very much of the rules from this great little book:http://www.amazon.com/Non-Designers-Design-Book-Typographic-Principles/dp/1566091594

Whoever designed that cover should be flogged.

Thanks Ari, nice find.