Temporary help agencies place nearly 3 million Americans in jobs each day -- but the temp industry's very success may embolden some managers to view all workers as impermanent, jobs scholar Vicki Smith argues in her latest book, "The Good Temp."

"Labor Day is an opportunity to remind ourselves that we have a long way to go to address the risks and vulnerabilities that workers face in today's global economy," says Smith, a professor and chair of sociology at the University of California, Davis.

In the "The Good Temp," Smith and her co-author, Esther B. Neuwirth, trace how temporary employment relationships have become mainstream in recent decades, and in some ways have contributed to the unraveling of the worker-employer contract.

At the same time, the authors argue that temporary help agencies have also had positive impacts, including providing training to temps and offering opportunities that may lead to permanent jobs.

"The Good Temp" is based on field work carried out in a temporary help agency in Silicon Valley.

Understanding the temporary help industry, its rise and the "good temp" worker it produces is important to understanding today's economy, according to Smith. She notes that only about 30 percent of American workers today have one permanent, Monday-through-Friday, 40-hour-a-week job, and that the underemployment rate -- the proportion of workers who are over-qualified for their jobs or are working fewer hours than they prefer -- has reached nearly 10 percent.

"Compared with the World War II era, when it was a marginal labor practice, temporary employment is today an entrenched feature of jobs and labor markets," Smith says.

Smith's previous book is "Crossing the Great Divide: Worker Risk and Opportunity in the New Economy." She is a past chair of the American Sociological Association's Organizations, Occupations and Work Section and of the Society for the Study of Social Problems' Labor Studies Division. She earned her doctorate in sociology at UC Berkeley.