What was the significance of Upton Sinclair's book The Jungle?
The main significance of The Jungle is that it changed the way many Americans came to regard the operation of the free market economy. Most people at that time took the existing system of capitalism for granted; it was just how things had always been. But in The Jungle, Upton Sinclair pulled back the curtain to reveal the rampant exploitation, the cutting of corners, and the indifference to public health and safety which the ceaseless drive for profit often promoted.
That's not to say that large numbers of Americans suddenly became converts to Sinclair's socialist ideals. The belief in the American Dream and the values of thrift, hard work, and private enterprise, were still firmly entrenched. There was, however, a notable shift in public opinion in favor of greater regulation of the economy. The lurid, stomach-churning depiction of Chicago's meatpacking industry, set out in such unerring detail by Sinclair, brought home to millions of Americans the stark realization that the market, left to its own devices, was incapable of meeting strict food safety standards. Hence the raft of Progressive legislation enacted in the wake of The Jungle's publication.
The American public continued to accept capitalism as the best economic system, the one more conducive to creating wealth and opportunity. But they now also accepted the necessity for proper rules and regulations to ensure that the system operated more fairly, for workers and consumers alike.