opus vs paperback

opus

noun
  • A work, especially of art. 

  • A work of music or set of works with a specified rank in an ordering of a composer's complete published works. 

paperback

noun
  • A book with flexible binding. 

verb
  • To issue or publish (a book) in a paperback edition. 

adj
  • Having flexible binding. 

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