It isnt. It isn’t widely considered a great book. Through at least Chapter 15 it is a work of pseudoscientific philosophy, and from 15 onward is adequate.  Mises’ reputation like that of most Jewish authors has been the subject of extravagant but unworthy promotion by Jewish Anarchists and a small number of third-tier academics who attempt to sway the unsophisticated with arguments that are ideologically useful but scientifically, widely if not universally rejected.  

The Austrian Christian movement through Hayek, has been fully integrated into classical economics, except for the open debate over the impact of various forms of monetary and fiscal policy on the business cycle.  The Austrian Jewish movement consisting of Mises and Rothbard, and to some lesser degree Hoppe, is widely considered a heresy or cult movement, and the mainstream has sought to distance itself from this rationalist and pseudoscientific fringe.

Prolific authors with activist supporters have spread Mises work as a mainstream alternative, to a population more able to grasp simplistic arguments rather that the heavily mathematical language of economics. Furthermore, the Mises Institute has used this work of pseudoscience as a means of raising money for over three decades and failed, to to more than expand the fringe group to autistically inclined, disenchanted males – a movement which has harmed the (Protestant) Classical Liberal Libertarian movement by damaging the brand ‘libertarian’ and associating libertarianism with fringe groups rather than the anglo saxon tradition of common law, the family, and self reliance, back into our ancient history.

Mises, like many of his contemporaries, correctly intuited that something was wrong with the direction of economic inquiry, but he, even less so than his peers in math and science, was unsuccessful in identifying it.

And instead he resorted to elaborate verbal pseudoscientific argument, unsupported by empirical evidence, to justify his preconceptions of how economics ought to work if it worked for the benefit of investors rather than the benefit of the commons (everyone). 

Mises is, like Rothbard and Marx, Freud and Cantor and in fact most of the Cosmopolitans, and no small number of the German Continentals, an authoritarian who will happily resort to pseudoscience and elaborate verbalisms to construct arguments that they cannot by scientific and demonstrable means.

This is the correct interpretation of Mises: as an advocate for investors who used pseudoscience to justify his preconceptions.

Economists don’t read Marx or Mises except as literary diversions. If you do choose to read Mises, read him as an author of cosmopolitan middle class pseudoscience the same way you read Marx as an author of lower class pseudoscience, or Strauss as an author of upper class pseudoscience.

But we appear to be coming to the end of a century and a half of pseudoscience – thanks to science. Particularly science since 2000.

https://www.quora.com/Why-is-Human-Action-by-Ludwig-von-Mises-considered-a-great-book