I'm learning economics, marketing and business as well and there are good resources online (although, that's true, I'm reading a few books as well, but it's the same for computer stuff in fact).

Among the resources I know and use are:

Paul Graham's essays, like many people here (I prefer reading them on paper anyway, but all of them are available on his website);

Joel Spolsky's blog (joelonsoftware.com) used to have very good articles (lately it's not as good as it used to be); he had the best of them published, too.

Eric Sink's blog (software.ericsink.com); great articles on marketing and business for geeks.

There are good articles about the basics of business in the Trizoko online business journal: http://www.trizle.com/trizoko

On changethis.com you'll find a lot of great PDFs on business subjects (very good resource; there is good stuff by Seth Godin here)

For economics, I'm reading the work of Frédéric Bastiat, available online here: bastiat.org.

Good old Bastiat... That's definitely fairly remote from "mainstream" economics these days, although there is a lot of sense to the proposition that the works of the classical economists are more practically useful than the abstract modeling you'd learn as an economics major. If you nonetheless want, for whatever reason, to learn something akin to the modern standard microeconomics curriculum, albeit with a good deal more game theory thrown in, I can recommend the free (Creative Commons!) textbook by Preston McAfee at http://www.introecon.com

Sure, that's all good stuff, but there's a difference between "reading" and "learning". The part of the learning process that works so wonderfully in the world of computers is that you can:

1. Read something

2. Try it out.

3. Ask questions of others about the results, then go read more, try it again, and so on.

Not quite so easy for economics, business and marketing.