It seems that, at least in my hometown paper, all movies are rated on a four- or five-star system, but books are given no ranking. Why aren’t books given stars? (I know they are rated with stars on Amazon by reader polls but I am wondering about reviews by professional reviewers in mainstream media.)
I would say it’s a high-brow/low-brow thing.
Movie reviews in the New Yorker and New York Times do not have starred ratings. Book reviews in Entertainment Weekly get a letter grade.
Books tend to be more complex then films.
Books are also more diverse in what they are trying to accomplish. If you’re reviewing an instructional book, a historical book, and a fiction book, does it really matter which one you enjoy most?