amass vs build up

amass

verb
  • To accumulate; to assemble. 

  • to gather a great quantity of; to accumulate. 

  • To collect into a mass or heap. 

build up

verb
  • To accumulate, to pile up, to increase in stages. 

  • To close up by building. 

  • To strengthen. 

  • In solitaire card games, to place a card over another card of lower value. (e.g., place 5♦ over 4♣) 

  • To erect; to construct. 

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