epistle vs letter

epistle

noun
  • A letter, or a literary composition in the form of a letter. 

  • One of the letters included as a book of the New Testament. 

letter

noun
  • A written or printed communication, generally longer and more formal than a note. 

  • A size of paper, 215 mm × 280 mm. 

  • Literature. 

  • A size of paper, 8½ in × 11 in (215.9 mm × 279.4 mm, US paper sizes rounded to the nearest 5 mm). 

  • A symbol in an alphabet. 

  • A division unit of a piece of law marked by a letter of the alphabet. 

  • One who lets, or lets out. 

  • The literal meaning of something, as distinguished from its intended and remoter meaning (the spirit). 

verb
  • To print, inscribe, or paint letters on something. 

  • To earn a varsity letter (award). 

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