DOWN AND OUT IN AMERICA The Origins of Homelessness <i> by Peter H. Rossi (University of Chicago Press: $15.95; 282 pp.) </i>

There’s no mystery about it: Homeless people are homeless because they are very poor. “Only” a quarter of a million Americans lack any shelter at all, says Peter H. Rossi, a sociology professor who studied Chicago’s homeless in 1986, but 4 million to 7 million others are so poor that almost anything can shove them over the edge: an illness, a fire, a rent hike, the snapping of already tenuous family ties.

“The pool of extremely poor persons that the literally homeless are drawn from has increased enormously since 1970,” Rossi says. The homeless themselves are poorer. During the post-World War II boom, the dwindling Skid Rows of U.S. cities held mostly elderly white men who lived in cheap hotels. Today’s homeless are younger and include more blacks, women and children. They find less work than their predecessors, earn much less money and, priced out of what little low-income housing remains, are much more likely to live in the streets.

This is a dry, scholarly study, complete with graphs and clubfooted prose, but Rossi is far from detached. He surveys the history of homelessness in the United States, summarizes other studies, defines his terms, fusses over methodology, explains how the difficult task of counting and interviewing the homeless was accomplished, corrects certain stereotypes (not all homeless people, or even most of them, are alcoholics or drug addicts or the mentally ill)--all in order to shake us out of our detachment. It’s useless to haggle over the exact number of homeless people, he maintains (though he can haggle with the best of them), if that keeps us from solving the real problem: poverty.

A solution can be reached, Rossi argues. Social Security and other programs have greatly reduced poverty among the elderly. We can do the same for the homeless and near-homeless through public-sector jobs, co-op housing and higher welfare benefits. Yet the elaborate documentation of this book is, in a sense, an acknowledgement that the will for such an effort is lacking. Policy makers, strapped for funds, would no doubt prefer that the homeless remain a mystery. Rossi tries to take away that excuse.