TRUMAN: The Rise to Power. By Richard Lawrence Miller. McGraw-Hill. 536 pp. $19.95.

AS WORD BEGAN circulating in wartime Washington that the hardworking Sen. Harry Truman of Missouri might be White House material, Truman himself wrote: "It scares me to death, the things they say to and about me."

But even by the demanding presidential standards of that day -- it was 1941 -- few men have undergone more exacting preparation for the great office. That has come to be more widely understood as the reputation of Truman's presidency has ascended. But just what the apprenticeship consisted of -- beyond the artillery captaincy in France, the bookishness, the failed venture as a haberdasher, the Missouri "judgeship" (a county administrative post) -- has been sketchy.

It need never be so again, now that we have Richard Lawrence Miller's account of Harry Truman's "rise to power." In lush and loving detail, which is only occasionally excessive, Miller presents a magisterial study of the texture of local politics in early 20th- century mid-America. It is a study which happens to have as its foci Harry Truman and Jackson County (Kansas City-Independence), Missouri, but in countless ways it richly epitomizes the lost worlds of the old politics.

When Harry Truman, farm boy and reader of heroic biography, was cutting his teeth as an apprentice Missouri politician, the usual avenues to "national" political prominence led by way of local and state office. In Harry Truman's case, which was typical, local politics were also intricately (and healthily) entwined with networks of business, social and spiritual camaraderie -- especially the National Guard and the Masonic lodge. Then, and yet today in some places, these and other associations formed the matrix of small-city life.

If this fascinating book had been called "The Structure of Politics at the Accession of Boss Tom Pendergast," with a nod to Sir Lewis Namier's great studies of the local basis of 18th-century British politics, the echo would not be misplaced. Miller boasts -- it is an unusual and refreshing boast for a political biographer today -- that he is "the son of a county patronage politician," and that his family's livelihood "depended on courthouse intrigue and electioneering." What is often evaded, in the sterile world of the new media politics, as something d,eclass,e and vaguely shameful, Miller celebrates; and this background has served him well indeed.

There can be very few unturned stones left concealing further information about the ancestry, family, courtship, finances, education, ambitions and friendships of the young Harry Truman. Miller has tracked them all down. And before he has done with the subject, you understand, moreover, how local elections were organized, managed and paid for; how patronage was distributed among the feuding and kaleidoscopic factions of Jackson County; what it meant (and didn't mean) to be a machine boss; and precisely how Truman, as an elected administrator in the mid-and late 1920s, dependent on the goodwill of the Pendergasts, served the machine and yet distanced himself from its sometimes flagrant corruption.

One is reminded, reading all this, that the political world in which Harry Truman grew up was in many ways closer to the Jacksonian age in American politics than to our own -- if by Jacksonian one means a world in which small entrepreneurs contended for a place in the sun against the storied financial, corporate and legal powers of the East. It is indeed true that Harry Truman, fresh back from France, entered a (brief) haberdashery partnership with his friend Sgt. Eddie Jacobson. Miller has added to the familiar tale of that socially and politically useful, but financially disastrous, two-year fling much else that Harry Truman tried: ventures in land speculation in Oklahoma and Montana, oil and gas leases, lead and zinc mining, savings and loan and banking operations. None made him rich; most gained him friends and admirers.

Indeed, having served two innovative terms as chief judge (administrator) of Jackson County, it was the height of the future president's political ambition -- or what he was resigned to, in the Depression years -- to settle into a job as tax collector. Money was to be made there; and Truman was very scrupulous about his debts, at least the ones he deemed legitimate. Then, suddenly, almost accidentally, the Senate seat fell to him from the hand of Boss Tom; and in 1935 he found himself in Washington as Senator Truman.

SOON, ALREADY versed as he was in so many entrepreneurial worlds, he joined Senator William Borah's investigation of railroad finance and within a short time his diligence had made him its star. His special targets were the holding companies which, through arrangements we today call "leveraged," controlled and milked the railroads. As World War II came, he progressed to the scrutiny of another complex world, the world of military contracting, ususally sticking up for the small manufacturer jostled by greedy giants. From there, as his reputation grew, it was only a short and natural step to the vice presidency. By 1943, it was known that FDR was failing, and Henry Wallace was not regarded by the party as a suitable heir. The rest of the story is far better known.

I have only a minor quarrel or two with this otherwise splendid book. Its promotional blurbs suggest, a bit misleadingly, that it is meant to be vaguely revisionist and even faintly sensational -- maybe taking the saintly reputation of Harry S. Truman down a notch or two. ("A lively portrait . . . at variance with the prevailing image," says one endorsement). This is nonsense, but it does reflect a certain ambiguity in the book's tone and theme: as if Miller could not quite make up his mind about the character of Harry Truman.

As Miller shows, Truman cut his corners on occasion, as a public servant; and only prigs and innocents suppose that unspotted alabaster is the stuff of which effective presidents are made. Yet Miller clearly likes his subject, and he repeatedly shows that when the chips were down Truman invariably made honorable and public-spirited decisions, in Missouri and Washington. Yet Miller can write, for instance: "In politics Truman viewed honesty and honor in terms of keeping promises. . . . In politics a man had to stand by his friends no matter what, even if the circumstances that formed the friendship radically changed, even if the friends were exposed as crooks."

Truman's dogged loyalty to old "cronies" who fell afoul of the law was, indeed, a standard reproach against him after he reached the White House. But the implication that this friendship-is-blind loyalty was the measure of his conception of public duty is pompous nonsense, and Miller should not connive in it, even by omission. It is repeatedly contradicted, in fact, by the evidence Miller has amassed. Harry Truman, though by origin a machine politician, and though often in debt, never stooped to the games of skulduggery and self-enrichment that were standard operating procedures in the world of machine politics.

Similarly, it is interesting to learn that Harry Truman, as a rising Missouri politician, came near joining the Ku Klux Klan, and it is hardly surprising that his casual racial attitudes and private utterances were conventional for his place, time and caste. What is far more significant is that the first presidential advocate of a real civil rights program could already be glimpsed in the Missouri politician who in 1940 said: "When we speak of man and his labor . . . we must consider the problem of our Negro population and bend our every effort that, at least under law, they may claim their heritage of our Bill of Rights. . . . I believe in the brotherhood of man; not merely the brotherhood of white men, but the brotherhood of all men before law. . . . The majority of our Negro people find but cold comfort in shanties and tenements. Surely, as freemen, they are entitled to something better than this."

I think Mr. Miller, having learned just about all there is to know of the early years of Harry Truman, owed his subject more careful and discriminating judgments than he sometimes renders. But that is a relatively minor complaint. Its flaws notwithstanding, The Rise To Power is a classic contribution to our understanding of a great man and the lost time that molded him and gave him to the nation.