This is how I rate books!
R- Recommended NR- Not Recommended
1/5 = Poor = NR
2/5 = Okay = NR
3/5 = Decent = NR
3.5/5 = Good = R
4/5 = Great = R
4.5/5 = Really Great = R
5/5 = Amazing = R
Here is a video to explain it further: