Chapterettes, Prologues, Introductions, and Other Spare Parts--What Purpose Can They Serve in a Book?

I happen to love small pieces of books:  prologues, introductions, forewords, even epilogues, and epigraphs (those quotes or small things planted before each chapter).  Such add-ons often get derided in writing classes, but they still serve a unique purpose. 

I fought one of my MFA advisers who hated the idea of a prologue in my young-adult novel, and w…