Movie companion books, as I personally define them, are books that are released in conjunction with (or after) the release of a movie, maybe even sometimes before. They typically add information about characters, plot, settings, or other aspects about the movie or the universe the movies was set in.
These books aren't necessarily 100% dependent on seeing the movie first, and can be good sources of information.
See my a question here: Why was the Obscurus omitted?. It's admittedly not a great question in itself, but that shouldn't be condemning for ALL companion books.
So, what do we think about companion books, or other pieces of literature written to accompany a movie?