Preface vs preceptor

Preface

noun
  • The part of the liturgy that precedes the main part of the Eucharist 

preceptor

noun
  • A doctor who gives practical training to medical students, nurses etc. 

  • The head of a preceptory of Knights Templar. 

  • A teacher or tutor. 

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