author vs ghostwriter

author

noun
  • Someone who writes books for a living. 

  • The originator or creator of a work, especially of a literary composition. 

verb
  • To create a work as its author. 

ghostwriter

noun
  • A professional writer who is paid to write material that is officially credited to another person; one who writes on behalf of someone else, often for a celebrity. 

  • Smith was listed as the author of his autobiography, but a ghostwriter did most of the work. 

How often have the words author and ghostwriter occurred in a corpus of books? (source: Google Ngram Viewer )