From the book’s Preface :-
In music, a ghost note is a musical note with a rhythmic value, but no discernable pitch when played. It is a subtle, subliminal percussive kick or short note, never notated in the score. In jazz it is often played in between the quarter note pulse in a random fashion.
A ghost is the soul or spirit of a dead person, that haunts particular locations or people they were associated with in life, that can appear – in visible form or other manifestation – to the living.