closed book vs mystifier

closed book

noun
  • A person or thing that cannot be easily understood; someone or something incomprehensible or puzzling. 

mystifier

noun
  • A person who obscures or muddles information, making something appear to be a mystery. 

  • A situation that is puzzling, or mystifying 

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