opus vs reference book

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. 

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 opus and reference book occurred in a corpus of books? (source: Google Ngram Viewer )