http://en.wikipedia.org/wiki/Tie-in In this case, it means it is a new version of the book with a new cover. The cover will generally be a picture of the actor/actress who plays in the movie version of the book. It is used as a way of promoting both the book and movie.

The noun "tie" has many meanings. I heartily recommend you look the word up in a good dictionary, because the word will be very useful to you in its many meanings.