So there are books that are made into movies. Quite often the original book is worth reading (though not always–Forest Gump the book isn’t great).
Then there are novelizations of movies. These are cheap and basically involve someone watching the movie and writing down what happens. For some reason my brother and I used to get a fair number of these things when we were young. I remember Back to the Future (and perhaps the sequels). They were not very good.