The Powers That Be, by David Halberstam. New York: Dell Books. $3.50.

Halberstam's study of America's massmedia giants -- including the TV networks , the New York Times, the Washington Post, and Time magazine -- is presented in four sagas, each nearly a book in itself. Though it has some faults, this is a reasonably full history; and for any study of the marketplace of information in America and the question of bigness as it pertains to the free, unbiased flow of news, this 1,000-page paperback will serve as an important source.