libretto vs sketchbook

libretto

noun
  • A book containing such a text. 

  • The text of a dramatic musical work, such as an opera. 

sketchbook

noun
  • A book of printed sketches. 

  • A book or pad with blank pages for sketching; a sketch pad. 

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