let off vs off the hook

let off

verb
  • To forgive and not punish. 

  • To cause to explode or come out; to release. 

off the hook

adj
  • Of a telephone, having an open connection; not hung up. 

  • Performing extraordinarily well. 

  • Relieved of a duty, burden, responsibility, or pressure. 

  • Fresh, cool, trendy, excellent, incredible. 

  • Of a telephone, ringing frequently. 

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