It's a dirty little secret in the publishing trade. Once an author achieves a certain level of prominence, at which everything he writes is certain to sell, his publisher will do a deal with him, and will hire ghostwriters to crank out books with his name on them. The author gets a major cut of the royalties for doing nothing. Sometimes the author has enough integrity to demand that the ghostwriter's name be put on the cover of the paperback, but often they do not.