Professional sports is a business, and with few exceptions everyone is out for what he can get."" Such is the attitude of Gary Davidson, the lawyer turned promoter who's the driving force behind the recently-formed World Football League. Davidson is no stranger to ""rebel leagues"" -- he was in on the ground floor of both the American Basketball Assn. (1967) and the World Hockey Assn. (1972) and served as president of both. He recounts here his wheeler-dealings with the ABA and WHA -- both are holding their own, though mergers with their established counterparts seem inevitable -- and his ""baby,"" the WFL. (Baseball owners needn't worry about future competition from him.) The facts and figures of franchise selling (supermarketing) and team trans. planting receive the most attention; the coups of Hull, Csonka and Kiick are downplayed. However innovative and dynamic Davidson may be, most readers will find the nitty-gritty of his new leagues infinitely less bouncy than that red, white and blue ball.