hardback vs opus

hardback

noun
  • A book with a solid binding. 

adj
  • Having a solid binding. 

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. 

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