[uzish]

Great question. The web being text oriented - I believe that typography is one of the most important aspects of a great website (visualization being the other).

If there's one seminal book - that would be The Elements of Typographic Style: http://www.amazon.com/Elements-Typographic-Style-Robert-Brin...

That's a deep dive into it though. If you're looking for a shortcut you can get a quick hold of them here: http://www.amazon.com/Fundamentals-Typography-Gavin-Ambrose/...

At Future Simple (http://www.futuresimple.com) we think about that a lot, sometimes agonizing over it. We still have a lot of work to do :) - as any designer will tell you - we are never satisfied with our work...