From my hagiographical piece at the American Thinker:
One of Mises’s earliest achievements was to bridge the two fields we now call microeconomics and macroeconomics.Originally, the classical economists of the eighteenth and nineteenth centuries had embraced variants of a labor theory of value in their teachings. Then, during the so-called Marginal Revolution of the 1870s, economists replaced the labor theory with the modern subjective theory of value, which sees all market prices as determined ultimately by the underlying preferences of consumers. It doesn’t matter how many labor-hours it takes to manufacture a product, according to subjectivism; if nobody really wants it, it will fetch a low price.
Economists gradually recognized the superiority of the new “subjective marginal utility” approach, but by the dawn of the twentieth century they still thought this worked only for “micro” explanations. The theory could explain, for example, how many bananas traded for how many apples, but economists still thought they needed an entirely different, “macro” framework to explain the money prices of goods.
Enter Ludwig von Mises. In his 1912 book, translated with the title The Theory of Money and Credit, he showed how to apply the theory of marginal utility to explain all market values — even the value of money itself. In so doing, Mises put individual money prices, and the purchasing power of money, under the umbrella of a unified theory of value.