Deceleration is a term used by some authors (including the author of your text) when the acceleration and velocity are of opposite sign; when this condition holds the object is slowing down.
I don't use the term, preferring to emphasize the fact that acceleration and velocity are both signed quantities and must be treated as such. I respect our author (obviously since I use his book), but in this case I agree with those authors who believe that we don't need two different words for the rate of change of velocity with respect to acceleration.