For fiction (and biography, in the rare instances I read it):
1 star: This book is terrible. It’s so bad I couldn’t even finish it to make fun of it.
2 stars: This is a seriously flawed story. It's poorly written, or its story logic is unbelievable and insupportable – but probably both are true. I'm a little sorry I took the time to read it, but it didn't make my eyeballs bleed. (That gets you 1 star.)
3 stars: This book is fine. It's not particularly masterful or moving or memorable – or maybe it is at some of its finer moments, but banal or incomprehensible narrative choices weighed it down and took me out of my enjoyment. Still, not the worst way to while away a few subway rides.
4 stars: I really liked this book. I want you to read this book so we can talk about it. It may not be a masterpiece, but it's got some great storytelling and scenes that I can't stop thinking about.
5 stars: I love this book so much that I want to marry its author and move into its pages to live forever in its world. This book may have changed my life.
For nonfiction, particularly scholarly writing:
1 star: This is bullshit.
2 stars: I didn't find this book interesting or useful. Possibility some of its methods or conclusions are flawed, or perhaps it's poorly put together – but most likely, both are true.
3 stars: This book is fine. I learned something from it, but I probably won't be spending more than a footnote on it.
4 stars: This book has been extremely useful to me, probably in both scholarly and personal writing. Maybe it's not as well-written as it is well-observed, maybe it's not so trenchant I would recommend it to folks outside of my research area, but *I* think it's pretty cool.
5 stars: Not only is this book interesting, thoughtful, and well-written, but it also changed my life.