Hmm. I'm willing to accept that the average reader is a more sophisticated and thoughtful consumer than for some other product categories. And there may be less overt tribalism and fanboyism in book reviews.
But, in my experience, there's still a lot of people who will almost unthinkingly give 5 stars to a favorite author or genre. (And I know some of those people who are pretty uncritical about SF books or whatever.)
Again, novels and products are entirely different in that you're hopefully shopping for a product by its specs. You can't 'research' novels beforehand (unless you count as reading other novels by the same author as research) and they don't have any specs to go by.
No, all assessments of value are subjective (and for most services involve subjective weighting of multiple different subjective quality factors.) And the mapping from an internal value judgement to a star rating is something which is a separate subjective evaluation, and one on which research has shown substantial cultural variation along ethnic and other lines.
The rating may be inaccurate because you can only measure in full stars, but it'll still be objective and the inaccuracy is the fault of the rating system, not me.
Objective (adjective):
> (of a person or their judgment) not influenced by personal feelings or opinions in considering and representing facts.
Perception of books and concepts covered within are entirely subjective.
However, when it comes to service or product quality, you can be almost entirely objective in its value judgment.