preface vs swan song

preface

verb
  • To give a preface to. 

  • To introduce or make a comment before (the main point). 

noun
  • An introduction, or series of preliminary remarks. 

  • A title or epithet. 

  • The beginning or introductory portion that comes before the main text of a document or book. 

  • The prelude or introduction to the canon of the Mass. 

swan song

noun
  • A final performance or accomplishment, especially one before retirement. 

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