In a recent Sunday NY Times, Henry Kissinger reviewed a new book about former Secretary of State Dean Acheson (Dean Acheson, A Life in the Cold War by Robert Beisner): Cold Warrior (New York Times, October 15, 2006).
The book sounds good ("sweeping and thoughtful account of Acheson’s tenure").
Kissinger says that the Secretary of State has five important tasks:
...Acheson dealt with the five principal tasks of any secretary of state: the identification of the challenge; the development of a strategy to deal with it; organizing and motivating the bureaucracy in the State Department and in other agencies; persuading the American public; and conducting American diplomacy toward other countries.