Truman's place in history; Harry S. Truman and the Modern American Presidency, by Robert H. Ferrell. Boston: Little, Brown & Co. 220 pp. $13.50.

Many people think of Harry Truman as ''a very small man who rose to the presidency by luck, by happenstance.'' By contrast, Robert Ferrell contends Truman will win a place in history as ''one of the best American presidents'' - decisive, modest, and an effective manager.

Ferrell, a History professor at Indiana University, traces Truman's political career from his first elected post as a Missouri county commissioner through his retirement from the nation's highest office.The atom bomb . . . the Truman Doctrine . . . the Marshall Plan . . . NATO . . . the Korean war . . . all emerged during his tenure as president.

In this concise, selective biography, Ferrell is clearly admiring but not uncritical. For example, he states flatly that the decision to bomb Hiroshima and Nagasaki was made ''with insufficient forethought.'' Yet the author's judgments are always well supported, including this one: ''[N]o political figure of the twentieth century . . . proved as willing as Truman to stand up for what he believed.''