libretto vs softcover

libretto

noun
  • A book containing such a text. 

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

softcover

noun
  • A book having such covers. 

adj
  • Having covers made of paper or thin cardboard; paperback. 

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