THE RIDDLE OF POWER: Presidential Leadership From Truman to Bush by Robert Shogan (E.P. Dutton: $21.95; 314 pp.). What makes an American President effective--able to become, in the words of Woodrow Wilson, “as big a man as he can”? That’s the question Los Angeles Times political writer Robert Shogan asks in this book, and answers by examining how three factors--ideology, values and character--have combined in the nine Presidents since Roosevelt to determine their administrative success. Shogan’s analysis is a bit schematic and deterministic--Why these three factors? Should they really lead us to label Truman, Kennedy, Nixon and Reagan as relatively “successful” Presidents?--but “The Riddle of Power” does illustrate the difficulties a modern President faces in attempting to implement an agenda in a constantly changing, often hostile political environment.