The Austrian economist Ludwig von Mises was arguably the most important thinker of the 20th century (or of all time, actually). His magnum opus, Human Action: A Treatise on Economics is a comprehensive and systematic treatise on economics, social philosophy, and the social sciences, and the foundation for sound, truly scientific economics.
Also groundbreaking and fascinating is Mises’ Epistemological Problems of Economics, originally published in 1933. In this book Mises seeks to put the declining classical view of economics as a deductive science on a firmer foundation, and to show why positivism and empiricism is the wrong approach to understanding economics. This treatise, out of print for many years, is now brought back by the Mises Institute in a 3rd edition, with a comprehensive introduction by Jörg Guido Hülsmann, senior fellow of the Mises Institute and Mises biographer. Hülsmann’s introduction is a brilliant tour de force, and well worth reading in its own right.