IN CASE it has gone unnoticed, which it has, the nation's capital is known in the poverty underground as a good tramp town. The police take no sport in bullying those at the bottom--either the migrant or permanent homeless. Meals are available at soup kitchens where no questions are asked and where the ladler may be an unprisoned criminal doing alternative service or a volunteer from the suburbs. In the summer, the parks are available for sleeping. In the winter, the city runs two shelters for the homeless; when that's not enough--when people begin freezing to death in alleys, as about a dozen citizens did last winter--a citizens group or two raises Cain to get more.

For most of those outside of Ground Bottom that's already more than enough to know about the skid row losers. What else is there to them?

For Leon Saunders, an Australian writer, and Douglas Harper, a sociology professor at the State University of New York, Potsdam, the lives of the homeless destitute became as worthy of study--and eventually, empathy--as any other fascination that comes across the sightlines of the curious well-off. Both authors display a willingness to get beyond the ambiguities of sociology and take their chances in the psychological worlds they enter and which are dominated by suffering that is often too primitive to convey.

"I never took much notice of the bums," Saunders writes about the homeless poor in Sydney, Australia. "But I could never quite bring myself to ignore them--most people seemed to. I could very easily see myself in their position in 20, maybe 10 years time, with a little twist of fate here or there."

Nearly all the twists that Saunders saw in the lives of Sidney's poor--Short Legs Dave, South Africa Bob, Len, the Baron, Panhandle Jack--were spun along with the help of alcohol. Eighteen of the 55 pages that contain photographs show men and women easing their addiction to the alcohol drug. Just as the affluent reject the company of the drunken poor, so do the besotted themselves. Saunders is told by one of the night residents at the Matthew Talbot Hostel for Homeless Men: "There's nothin' worse, if you're a drunk, than havin' a whole lot of other drunks around you all the time. It's bad enough at night time; they throw themselves around in bed, and they're yellin' out in their sleep an' all that . . . In the day time I like to get away from 'em."

The value of Saunders' investigation into the lives of the "shadow people" is that it gives public attention to private misery that is private only because this is a population with few allies among the powerful. Epidemics of physical and mental diseases are raging among the homeless, but neither medical schools nor medical literature mention homelessness as a health care category. The dead destitute get better attention in the morgues than the living in the streets.

The company kept by Professor Harper was the vagabond poor who ride the rails as freight-train migrants. During his travels in the boxcars of the Burlington Northern from Minneapolis to the apple town of Wenatchee, Washington, Saunders learned that this was a world of cultural distinctions. Tramps see themselves in a station well above bumland. Bums have lost control and will never rise above skid row.

Tramps work, bums beg. Deprivation has its etiquette. If a tramp accepts a handout--a sandwich, a swig--from another tramp, it must be returned in kind somewhere down the road.

The professor took for his teacher a tramp named Carl, a well-traveled man who knew the hobo jungles of the northern United States as trailer-tractor men know the truck stops. "As Carl's buddy, sidekick, or pupil," Harper writes, "I had begun to see the inner workings of a life I had only before observed on the surface. The understanding of the life included an appreciation, first, for the purposefulness rather than aimlessness that lay at the basis of the movement, the work, and even the drinking. These men were not 'drifters.' The life was a complicated, even intricate, set of rules and expectations, and the different ways in which different men on the road followed the prescribed ways of behaving layered them into distinct groups."

Of the two books, Harper's is the fuller and is likely to be around longer as a resource work. In his gunnysack, he kept a place for scholarship. He observes that tramps are disappearing--not because we are eliminating poverty but because, among other reasons, end-of-the-line jobs like apple picking are being displaced by apple picking machines. Another reason is that the railbeds are in such deterioration that the rides are often too punishing.

But before they vanish, Harper wants us to know that tramps have been "organic to the American social landscape," even if their presence both troubles and amazes us: "While (a tramp) represents the threat posed by those who do not take the conventional roles offered by society, he is also somewhat of a hero in his rejection of the routine, regulation and boredom of a life lived in the carefully protected niches of an increasingly organized society."

The book on skid row is thin, the one on tramps thick. In 10 years, perhaps sooner, it may be the reverse, as other writers continue the investigation of the fringes; the hobo jungles are disappearing and the urban jungles growing. Younger men and women are turning up at the soup kitchen. Should writers want to listen, the stationary urban homeless are waiting to talk.