opus vs softcover

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. 

softcover

noun
  • A book having such covers. 

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

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