Get ready for a lot more of heavyweight champ Mike Tyson in your neighborhood bookstore and/or on your TV set in the near future. Tyson has been signed by the William Morris Agency and will be personally represented by Morris co-chairman Norman Brokaw--who's helped past athletes such as swimmer Mark Spitz and diver Greg Louganis score big-time media gold. Brokaw said Thursday he sees development of a book, more commercials and a possible Saturday morning TV series as definite areas to explore for Tyson. "He's already been in the entertainment arena for years now," Brokaw said, "so this transition is a natural one. Mike's especially good at working with kids, so the Saturday morning show is especially attractive."