I grade books like Roger Ebert graded movies: compared to expectations.
If I am given an acclaimed classic like A Canticle for Leibowitz, and it ends up claiming babies suffering agonizing deaths are a good thing because god likes to see them go through it, and has no moral problems with that, even proposing that as the superior moral position, the grossness of its ideology makes it hard for me to ignore that the book's argument is also crap, so 1 star.
On the other hand, you have books like Meta. Classic? No. Acclaimed? No. Fun? Sure. Badly written? No. A quick read that leaves you wondering what happens next? Yes. So 3 stars.