It's a bit flaky, but I think that most fonts have a 'sweet spot' when presented on the web. For example, personally I think Georgia is great @13px or larger @6

It's a bit flaky, but I think that most fonts have a 'sweet spot' when presented on the web. For example, personally I think Georgia is great @13px or larger @60px but in between, it sucks. So try to pay attention to finding the right size for each font. But I agree with aba_sababa - you can learn specific technical details about fonts and typography in books, but an overall sense for good design has to be assimilated over time. So, when you see sites that you like, use Firebug and copy their font settings. Or if you see a site that has crappy typography (eg. Hackernews) then use firebug and try to fix it.