Dean Acheson: casual and caring; Among Friends: Personal Letters of Dean Acheson, edited by David S. McLellan and David C. Acheson. New York: Dodd, Mead & Co. $17.95

This collection of personal letters is pure Acheson, epitomizing that self-confident but also self-deprecating observer of the contemporary scene whose particular line of vision was within the framework of the 20th century yet curiously above it. Dean Acheson was President Truman's third secretary of state, sharing with his immediate predecessor, Gen. George C. Marshall, the credit for formulating the postwar and cold war policies of the United States.

These letters disclose an Acheson who was observer rather than policymaker, an Acheson who was casual, friendly, understanding, and caring. In his own inimitable style he fulfilled the roles of lawyer, public servant, husband, father, grandfather and elder statesman, plus dozens of other minor parts, in the course of a full and satisfying life.

The early correspondence reveals the young Acheson attempting to determine the appropriate path to a successful and challenging legal career. The editors then draw selectively from his voluminous correspondence of the '30s, '40s, and early '50s, the years when he was alternately in private practice or holding a government post. But the focus is on the years of so-called retirement when he was practicing some law, accepting assignments from the Eisenhower, Kennedy, and Johnson administrations, and furiously writing books and articles about his years of public service and about the contemporary problems of the '50s and '60 s. It is the letters of these last two decades of his life that most clearly reflect Acheson's perceptiveness and his unerring good judgment.

The man -- not the statesman -- dominates this book. The delayed birth of a grandchild in 1960 inspired him to observe to Archibald MacLeish: "I would point out that any child as bright as one of Mary's [his daughter] and Bill's, being urged to enter this world at this time would be pretty hard to coax down out of the high branches . . . . At any rate he, or she, won't show up -- probably sitting out the election." An announcement of the forthcoming marriage of Margaret Truman brought to the former President this warning: "All in all, the father of the bride is a pitiable creature. No one bothers with him at all. He is always in the way -- a sort of backward child -- humored but not participating in the big decisions." The dry wit, the charm, and the warmth of the man radiate from every page.

As one might suspect, there is an extended correspondence with several very special people. The most revealing letters are the letters to his "Boss," the affectionate title he used when addressing Truman. The most touching and candid correspondence is with Supreme Court Justice Felix Frankfurter and with Archibald MacLeish, two lifelong friends and confidants.

No sooner is the reader captivated by the man than he realizes that woven into these casual letters are some fundamental convictions about the way America should respond to the world and its problems. Acheson was a person who cut through the rhetoric of 20th-century diplomacy. In his world view, Russia was not to be trusted no matter how visible the signs of detente. He also believed Europe, not Southeast Asia, was where America's diplomatic and military engines should be focused. Even the legitimate aspirations of third world countries for self-government, he felt, should not be allowed to divert America from those problems that represented a direct threat to world peace. Read in the light of today's events in Iran and Afghanistan, his observations -- even if one does not wholly agree with them -- sharpen one's perspective.

Dean Acheson's adult life spans the years when America shifted from the august role of defining the world's priorities to the humbler one of simply trying to salvage them. These letters suggest that somehow he seems to have sensed the significance of the change better than anyone. And he certainly has expressed himself in a voice and style distinctly his own.