ghostwrite vs author

ghostwrite is not a noun while author is a noun.

Word	Noun	Adjective	Verb	Adverb
ghostwrite	No	No	Yes	No
author	Yes	No	Yes	No

As verbs, author is a hypernym of ghostwrite; that is, author is a word with a broader meaning than ghostwrite:

ghostwrite (noun)	author (noun)
	writes (books or stories or articles or the like) professionally (for pay)
	someone who originates or causes or initiates something

ghostwrite (verb)	author (verb)
write for someone else	be the author of