libretto vs reference book

libretto

noun
  • A book containing such a text. 

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

reference book

noun
  • A book providing factual information. 

  • A book that can be consulted in the library but cannot be borrowed. 

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