I use a five point scale, with arbitrary precision.
I usually try to work in increments of 0.5, though I occasionally dip into 0.1.
Roughly speaking, you can interpret the ratings as follows:
5 Stars - ⭐ ⭐ ⭐ ⭐ ⭐
These are works that I consider must-reads by everyone. A book that earns five stars in one that completely gripped me and something I intend to re-read multiple times.
4 Stars - ⭐ ⭐ ⭐ ⭐
A four star book is something that I am comfortable recommending to anyone, but not something that I can see myself going back to re-read.
3 Stars - ⭐ ⭐ ⭐
Books that are good enough that I finish them, but not impressive enough that I’m willing to recommend them to everyone earn three stars. These books enjoyable, but have readily identifiable flaws that impacted my reading experience.
2 Stars - ⭐ ⭐
It was a struggle to finish, or I gave up partway through. A two star book may possess some redeeming qualities, but they’re outweighed by the greater blemishes. Others may find them enjoyable, but it wasn’t for me.
1 Star - ⭐
Did not finish, these books are considered irredeemable and I would recommend you actively avoid them. I struggle to imagine why someone would enjoy any book that receives one star.