I try to be consistent when reviewing books, and think about ratings in a similar way to Net Promoter Score — how likely would I be to recommend this book to a friend? My ratings skew lower than what you might expect to see on Goodreads.
I consider books with this rating foundational knowledge. I reserve it for seminal works in a field, literary classics, or books that have significantly changed my behavior in some way. These books have stood the test of time.
I would strongly recommend this book. I often put exceptional books written in the last 5 years within this category.
This book is solid, but likely doesn’t have mass appeal. I would recommend it to a friend if they were interested in the topic area. This book may have flaws (e.g. unnecessarily long, a misguided chapter, etc.)
This book has one redeeming quality, but is likely not worth purchasing.
I would actively discourage friends from reading this book.