Economics is about making decisions when your decisions have trade-offs because you have limited resources. Everything has a price, but the prices aren't always measured in a nice money-based price tag for your convenience, they're ultimately measured by the next-best alternatives. Decisions and policies don't just have consequences, they have second-order effects and third-order effects and you need to chase those down too, because excluding anything makes you understand the costs wrong (e.g. add highway lanes to relieve congestion -> now there is more traffic, because of "induced demand", not sure that was a good idea -> but why was there even induced demand? where are those trips coming from and what was the cost of the previous configuration? is this a housing-policy thing now??). You also learn to worry a lot about how decisions are actually made by self-interested decision-makers, rather than by abstractions-seeking-the-greater-good, and you examine what the decision-makers' incentives are....
Whereas a work like Das Kapital tells you very little about how to think this way. It's not intended to; it's more about the relative influence of various actors within a political-economic system, while raising tough questions about the meaning of private property.