author vs singer-songwriter

author

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

  • Someone who writes books for a living. 

verb
  • To create a work as its author. 

singer-songwriter

noun
  • A musician who both sings and composes, especially when recording or performing his/her own compositions. 

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