opus vs textbook

opus

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

  • A work, especially of art. 

textbook

noun
  • A coursebook, a formal manual of instruction in a specific subject, especially one for use in schools or colleges. 

adj
  • Having the typical characteristics of some class of phenomenon, so that it might be included as an example in a textbook. 

  • Of or pertaining to textbooks or their style, especially in being dry and pedagogical; textbooky, textbooklike. 

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