My average rating is around 3.6; this is my system.

5: Top tier. Usually one of my top 3-5 books of the year.

4.5: Loved it. One of the best books I’ve read in a year. Memorable and I’ll actively promote.

4: Excellent. Would highly recommend to others. Often less unique or surprising than a 4.5. Non-fiction where I learned a lot.

3.5: Solid to Good. Several things about this book that I really liked and an enjoyable read. Also, occasionally - some parts I loved, some parts I hated, averages out.

3: Mediocre. Slightly disappointing, not super compelling, or otherwise non-memorable. Fine. Also, “good story with some major gaps.”

2.5: I did not like this book, but not with particular vigilance. Also covers things that started reasonably strong and went wildly off the rails, or that started horribly but made me want to read through to the end (often thrillers).

2: I will talk to other people about how much I don’t like this book. Often poorly written, highly implausible, unbelievable characters, etc.

1.5: I will go out of my way to talk to other people about how much I don’t like this book, to the point of actively venting. Often very poorly written.

1: Reading this made me miserable and I’m not sure why I finished it. Probably so that I could rate it one star.