ghostwriter vs letter writer

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. 

letter writer

noun
  • A person employed to write letters for illiterate people; a person who does this as an occupation. 

  • A correspondent; a writer of letters. 

  • A book containing examples of letters that can be imitated by those who wish to write correspondence. 

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