Ghostwriter

When a book or series is said to have been written by a person, but has actually been written by another person or even several people, it is ghostwritten, IE, has been written by a ghost. These actual authors are not ghosts except for the fact that they are considered invisible and that for general purposes, the person who is said to have written it is the actual writer.

Ghostwriting is not the same as plagiarism or using a penname.