BASEBALL; Fast book on Fernando

As far as Bantam Books is concerned, rookie pitcher Fernando Valenzuela of the Los Angeles Dodgers ranks right up there with "The Pentagon Papers" and "The White House Transcripts." Bantam already has a book out on baseball's prestrike sensation, the 70th such "instant" volume it has published since coming out with the Warren Commission Report in 1964.

Though newspapers and magazines have been glutted with stories on the 20 -year-old Mexican, Bantam obviously figured the public wants to know more about the pudgy-looking southpaw. So Mike Littwin, a Los Angeles Times sports writer, was put to work writing "Fernando," a dual-language biography with the English and Spanish texts under one cover. (Valenzuela speaks almost no English.) The paperback has hit he stands in the United States as well as Mexico and much of Latin Ameri ca.