Cf lukeprog's answer to my comment, indeed, much more people see movies - so when you generalize from fictional evidence, you'll refer to movies more easily than to books, because you've more chance of the other knowing the movie.

But you know that your audience did read a book, it's not more uncommon to call to a book (LOTR, Foundation or 1984 for example) than to movies. For myself, I tend to refer to books more than to movies when I know that people did read them, but more to movies when I don't know my audience well.