author vs ghostwrite

author is a noun but ghostwrite is not a noun.

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

As verbs, ghostwrite is a hyponym of author; that is, ghostwrite is a word with a more specific, narrower meaning than author:

Other hyponyms of author include co-author, ghost.

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

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