den vs spook

den

noun
  • A squalid or wretched place; a haunt. 

  • Synonym of fort (“structure improvised from furniture, etc. for playing games.”) 

  • A comfortable room not used for formal entertaining. 

  • A group of Cub Scouts of the same age who work on projects together. 

  • A small cavern or hollow place in the side of a hill, or among rocks; especially, a cave used by a wild animal for shelter or concealment. 

verb
  • To ensconce or hide oneself in (or as in) a den. 

spook

noun
  • A ghost or phantom. 

  • An undercover agent or spy. 

  • A hobgoblin. 

  • A scare or fright. 

  • A psychiatrist. 

  • A player who engages in hole carding by attempting to glimpse the dealer's hole card when the dealer checks under an ace or a 10 to see if a blackjack is present. 

  • A metaphysical manifestation; an artificial distinction or construct. 

verb
  • To frighten or make nervous (especially by startling). 

  • To become frightened (by something startling). 

  • To haunt. 

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