A cop story that gets behind the statistics on illegal immigration; Lines and Shadows, by Joseph Wambaugh. New York: William Morrow & Co. 284 pp. $ 15.95.

Illegal immigration, in the context of the ongoing debate in Washington, is a story of economics and politics - easy to tune out. ''Lines and Shadows'' is a story of human dimension that won't let a reader forget, or come away without an opinion about, the dilemma over America's southern border.

Joseph Wambaugh, a former Los Angeles policeman, writes cop stories. They are not pretty, but they have earned him a reputation for integrity in the law-enforcement community. ''Lines and Shadows'' is a true story about a controversial police experiment. Although the book explores controversies at several levels, its primary importance derives from the comment it makes on one of America's biggest social problems.

The economic vacuum to the South pushes people toward the United States' southern border, and this is the subject of a growing genre of storytelling, like the movies ''El Norte'' and ''The Border.'' If harsh language and some violence won't distract a reader from the essence of the book, then Wambaugh's story is the best done on this subject so far.

Silent masses of people, thousands every night, traverse the dusty canyons and that imaginary line dividing Mexico from the US. These pollos are quiet and docile because they are illegally crossing north to perceived prosperity, and they must avoid the twin dangers of la migra (US immigration officers) and bandits who ply the canyons. Many of San Diego's murders, rapes, and robberies occur in this dark border zone. The victims are exclusively aliens, vulnerable targets who carry their life savings in their pockets and are not likely to report crimes if it would mean being sent back to Mexico. The assailants are the victims' Mexican countrymen, who know that the penalties for rape, robbery, and murder are stiffer on the Mexican frontier than just across the border. So between 1976 and 1978 the San Diego Police Department fielded a foot patrol to deal with the problem.

Wambaugh documents the course of this border-crime task force from ridicule in the beginning, to local hero-worship at their peak, and finally to the group's disintegration after ''gunslinging'' excesses drove the members to other jobs or psychological despair. The task force was conceived and ramrodded to reality by Lt. Dick Snider. His theoretical basis was altruistic, and Wambaugh makes it all too clear that this vision was lost in bureaucratic politicking and personal problems. That turning point in the story comes when the task force is handed over to Sgt. Manny Lopez - a go-for-broke kind of cop who was idolized by his younger charges and viewed as a raving maniac by police veterans. He was the kind who would go for his gun even when a bandit had the muzzle of a gun in Lopez's face.

Although it was clear that the mission of the task force was morally right, a shadow of doubt always hung over its purpose. These cops were largely protecting illegal visitors who were being victimized on the American side of the border by equally illegal visitors. And that was controversial, even if it was morally right.

These gonzo cops were getting a lot of publicity crawling around among the snakes and tarantulas of the canyons dressed as alien decoys. Their gunslinging was so popular that residents on the other side of the canyons in the Tijuana shantytown of Colonia Libertad applauded when they heard their gunshots.

Ultimately the task force experiment ended, a victim of its own zeal. But the victimization of aliens continues.

Perhaps the most telling reason for reading ''Lines and Shadows'' is expressed by former task force member Ernie Salgado when he remarks, ''Until I was on the task force, I really didn't understand their plight.''

Up close, he says, the motives and vulnerability of the aliens become compellingly clear. That's why a reader is likely to extract more understanding from ''Lines and Shadows'' than from all the headlines, statistics, and political haggling on the subject of illegal immigration.