I like Peter's comparison of a ghost writer to a character actor who is able to adapt the voice and personality of many different people and somehow seamlessly convey a sense of their inner life through something more than mere mimicry--as contrasted with the star actor whose own personality is so intense he "plays himself" in every role, just wearing different costumes and uttering different lines. The true ghost finds self-expression through self-abnegation--paradoxical, but when it works, it works.
Second, just for fun, this archive of historical novels (and other kinds of books) in which the same work of art has been recycled once, twice, even several times to illustrate the covers of different books. If you think the jacket of some newly published book looks weirdly familiar, it's probably not your imagination . . .