I feel like Wealth of Nations probably tops Piketty's Capital in the Twenty First Century, as does On the Principles of Political Economy and Taxation by David Ricardo, The Theory of the Leisure Class: An Economic Study of Institutions by Thorstein Veblin, The Road to Serfdom, Individualism and Economic Order by F.A. Hayek, Human Action by Ludwig von Mises.
Definitely You should read "The general theory of employment, interest and money" by John Maynard Keynes. And from the other side of the economic spectrum try: "Free to choose" by Milton Friedman. Probably, you should also pick: "This time is different" by Ken Rogoff and Carmen Reinhart.
Wealth of Nations is old, all the way back from 1776, but many of its simple points on the nature of capitalism still ring true today. It predates the math-ification of economics, so it remains reasonably accessible to the lay-man (if I can read it, you definitely can)