Mirror Mirror: The Movie Novel and Mirror Mirror: The Movie Storybook are already available so you can read and "spoil" the movie for yourself. Somehow--and this is not cynicism--I am not expecting many surprises in this movie so I doubt there is much to be spoiled. I feel like the previews have covered the material excepting perhaps a few jokes.
These are aimed at children--the publisher is Scholastic after all. So these might work especially well with difficult readers as so many movie tie-in books do. If children are reading, I am pleased. I hope many of them fall in love with this movie since it bodes well for the fairy tale's future!