letters vs printed matter

letters

noun
  • Literature (school subject). 

  • The liberal arts, humanities, learning (broad accumulated cultural knowledge). 

printed matter

noun
  • Printed material produced by printers and publishers, including books, magazines, brochures, booklets, publicity material and newspapers, especially in relation to cheaper postage rates on such material. 

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