We’re having the wrong conversation about higher education in our country. So argues Suzanne Mettler in her ­provocative new book, “Degrees of Inequality,” based on the eight years she spent studying a college system that she argues works well for those born into well-off families but few others. Tuition rates of $50,000 or more at private four-year colleges? The trillion-plus dollars Americans collectively owe on college loans? In ­Mettler’s ­telling, those should be the least of our worries when considering the “caste system” that is now higher education in the United States.

The picture Mettler offers of the ­postsecondary-education landscape in the United States is not pretty. Looking past the top stratum of elite colleges that normally dominate discussions of higher education (Mettler herself is a professor of government at Cornell University), she chronicles the deterioration of the country’s once-vaunted state college system, where a majority of students pursuing a postsecondary degree are enrolled. She bemoans the fact that the community ­colleges, which play a central role in educating the “less advantaged,” must beg for money, and she lays into for-profit ­colleges like the University of Phoenix, the largest of the 1,000 or so of these institutions that have sprouted up in recent years.

For-profit colleges are the true bad guys in this tale. Though their “ardent defender,” the Republican Party, contends that the schools provide “meaningful opportunities for low-income and minority students,” Mettler mounts a persuasive case that something like the opposite is true: These institutions are generally more skilled at getting rich off those living in the lower economic reaches than they are at preparing them for the job market. She has mined congressional reports, newspaper accounts and academic ­studies, piling up example after example of recruiters who’ll say practically anything to enroll a student, any student, in their programs, resulting in graduation rates not even close to those of traditional colleges. And how do those who manage to earn a degree fare? In the 2007-8 academic year, the average student working on a bachelor’s degree from a for-profit college found herself in deeper debt ($32,700) than her counterpart attending a private college ($17,700). And good luck settling loans with those low-paying jobs so many graduates find themselves working, despite the dreams that the school’s marketers put in their heads. Alumni of the for-profit colleges account for nearly half of all student-loan defaults, according to Mettler, even as they make up one in 10 students pursuing a postsecondary education.

How did the institution that we would call career college or trade school transform itself into a $32-billion-a-year business? Mainly by “milking” the government for profit, and taking advantage of student-loan programs designed to help those on the bottom of the economic pyramid continue their studies beyond high school. The Apollo Group, which owns the University of Phoenix, derives almost 89 percent of its revenues from the federal government, and Kaplan Higher Education, owned by Graham Holdings (formerly the Washington Post Group), almost 88 percent.