The Hardy Boys and Nancy Drew series were written by many authors under a collective pseudonym from the beginning. I'm beginning to think James Patterson's volume of work indicates something approaching that level, too. The romance genre seems to be rife with pseudonyms. Nora Roberts is the same author as J.D. Robb. Amusingly enough, even though Jayne Ann Krentz is Jayne Castle is Amanda Quick, I have had readers emphatically state preference for her various pseudonyms, perhaps due to the different subgenres they represent.