Unsympathetic biography of U.S. Grant; Grant: A Biography, by William S. McFeely. New York: W. W. Norton & Co. $19.95 .

James A. Garfield, who would later become president, said it all: "His imperturbability is amazing. I am in doubt whether to call it greatness or stupidity."

The occasion was Ulysses S. Grant's casual acceptance of the frantic resignation of his secretary of war, William Belknap, in March of 1876 after irrefutable evidence of Belknap's bribe-taking became public. But the observation could have been applied to most of Grant's life with the same validity. Every student of Grant understands Garfield's dilemma; William S. McFeely constructs a biography around it.

It is not easy to write the biography of a man who was so marked by ignorance , naivete, and ineptitude, a nobody, propelled into power and influence despite himself. Grant was a total failure at almost everything he did except waging war, and in that he was merely determined, more noted for his appalling casualty lists than his clever tactics.

McFeely knows his man, the enthusiastic junior officer in the Mexican war, the disgruntled captain in the peacetime army who ultimately resigned, the fumbling businessman in the 1850s, who made a spectacular rise to national prominence because the Civil War permitted him to do what he was most qualified for -- wage war.

This becomes the story of a man who, quite literally, was saved from poverty by the war and was afraid to have it end because he had no idea what to do with his life once the fighting stopped.

But as we know, Grant did not sink back into obscurity.He believed in himself with a certainty that he would make his mark, if only the right position became available to him."He wanted to matter," said McFeely, "in a world he had been watching closely all his life." The position, unfortunately, was the presidency, willingly bestowed on him by a grateful but unthinking nation.

Only three issues really interested him during his eight years of office: He wanted to help the American Indian; he was obsessed with acquiring the island of Santo Domingo; and he assumed that the program of Reconstruction passed by Congress ought to be carried to its logical conclusion. But to all else, save the cheers of the crowd, he was oblivious.

The way Grant's presidency is presented in this biography leaves the reader not so much outraged by the numerous scandals and wholesale corruption of his administration as wondering how such a dangerously innocent man was ever able to reach the presidency.

Most of us know Grant as a reasonably effective military commander and as one of our worst presidents. McFeely does not so much reverse these assessments as recast them. And he does so with such acroitness that the reader finds himself repeating the standard judgment with a new vocabulary. Grant was one of the most effective Civil War generals -- because he took if for granted that heavy casualties were virtually inevitable in any major campaign. Grant was one of our worst presidents -- because he could not begin to comprehend how utterly vulnerable he was to exploitation. McFeely simply makes us say: "Poor fellow, he probably meant well!" And that is biographic al technique at its best.