A DEBT-STRAPPED farmer walked into his small-town bank near Cedar Rapids, Iowa, last December, pulled out a gun and shot the bank president. The killing was page-one news in The New York Times, reported by its Chicago bureau chief Andrew H. Malcolm.

Oddly enough, Malcolm had just finished writing a book about how another small-town bank president, in southern Minnesota two years earlier, had been shot dead under similar circumstances.

Final Harvest tells how Rudy Blythe, a former Philadelphian who has bought a bank in Ruthton, Minnesota, and Toby Thulin, his chief loan officer, are led into an ambush on September 29, 1983, at an abandoned, overgrown homestead. The land, just 10 acres, has been epossessed. Lying in wait are its former owner, James Lee Jenkins, a sociopathic loner, and his 18-year-old son, Steve, a shaven-headed, rifle-toting, tattooed gun freak who goes around in combat fatigues. Shots ring out, killing the two bankers. The Jenkinses flee to Texas, where the father takes his own life. Steve Jenkins gives himself up, is tried and found guilty of first- degree murder.

Malcolm is an able journalist and a good storyteller, with an obvious feeling for his subject and compassion for his characters. His narrative is well-paced and makes a suspenseful true-crime story. At a time when so many farmers are on the brink of financial ruin, Final Harvest also adds substantially to our understanding of the rage and fear people feel when they mistakenly blame themselves or others for what are really society's failures.

Since the growing plight of American farmers has sown a whole new genre of save-the-farm films, Rudy Blythe and James Jenkins seem fated to appear on our living room TV screens. Malcolm's publisher goes so far as to claim such killings are "dramatic evidence of the death not only of individuals but, in large part, of the small-town American farmer." If so, we all have a lot to worry about.

Malcolm himself puts it perhaps more accurately. In his December story on the Minnesota murder (by an elderly farmer who was overdrawn at the bank), he described what he called "a fundamental restructuring across the country's midsection, which historically has produced so much of the nation's food and factories, its leaders and social values." Ever since John Deere's steel plow and John McCormick's reaper made it possible to farm the Great Plains, there have been gutsy, poor farmers struggling to preserve their land from the men at the bank. What is new is their numbers. The mid-19th-century shift from human to horse power led to a steady increase in the number of farms, from 1.5 million in 1850 to 6.8 million by 1935. This number started to fall once farmers went from horses to tractors, mainly during World War II, after the application of chemistry to farming in the 1930s (nitrogen fertilizer) and biology in the 1930s (improved seed and livestock). Today we are down to 2.2 million farms. Something like 5 percent went under last year.

The immediate problem is that farmers borrowed too much during the export boom of the 1970s, when land prices were inflated, to hope to repay the loans. Debts mount and many are threatened with foreclosure. Often the smallest farms are hardest hit.

A more enduring change is that we have taught the world to grow its own food. Moreover, our biggest gains in production have come from oil-based mechanical technology, advances in output per worker. Put a teen-ager in a $100,000 combine in North Dakota, and he can harvest 12 acres of wheat in an hour. The Europeans and Asians are starting to outproduce us with biological technology -- seeds, water and fertilizer -- or advances in output per unit of land.

Economists say it is getting too expensive to subsidize the American family farmer just to preserve our rural values. The truth, as Final Harvest illustrates, is that rural values -- like respect for property or the place given marriage and the family -- just may be our only values. As history shows, every society goes into decline, however slowly, once it gets too far away from its agricultural origins.

So that as a social document, Final Harvest is undoubtedly a book that is going to matter. When it comes to farming, Malcolm clearly knows his stuff. He has some nice detail:

". . . Set the alarm for three each spring morning to check the cows for birthing problems, especially the first-time mothers. Watch them eat more grain the day before a winter storm as if they knew, which they do. Be alert for those little mood swings, droopy ears or tail, dull eyes . . ."

This is the remembered observation of somebody who has acually done chores.

The book is skillfully organized, and Malcolm generally choreographs successfully the conflicting views of victims and killers and later the opposing legal sides in court. The occasional shift into present tense seems a needless stylistic flourish, though, and a couple of literary allusions (John O'Hara, Lewis Carroll) ought to have been discouraged.

IF FINAL HARVEST has one significant, perhaps unavoidable, flaw, it is that we get to know the victims much better than the killers. The reader gets so caught up in the gathering hysteria of the likable banker, Rudy Blythe, and especially of his well- portrayed wife, Susan, that the shooting scene out in the misty rain is terrifyingly real. James and Steve Jenkins appear as often in the book's pages, but somehow they fail to make their presence come alive in the same way.

Once the author writes of them: ". . . some people were saying Steve was an oddball son of a sick father." That is exactly how the reader has come to see them too. Honestly sweating, laboring men, maybe. Downtrodden, perhaps. Wholesome and virtuous, definitely no. The down-home image of this seedy and unstable father and son is badly tarnished. It is they, not the bankers, who emerge as the villains of the piece.

This gives Final Harvest a moral ambiguity and sense of reality quite different from the wave of farming films. It obviously posed a problem for Malcolm. He has done his homework well, but especially in the character of Steve we are left with a shadowy, sinister, slightly pathetic figure. It comes as a surprise when the defense attorney and his wife grow so fond of him they formally adopt him during the trial.

The killer's own voice is missing, and at the end of the trial, as he is led away sobbing, the reader is left a bit puzzled and dissatisfied. One suspects this was Malcolm's feeling too.