Amazon.com Rating Scale
5/5 - pure genius, excellence, the best of the best (Examples are The Poisonwood Bible, The Kite Runner, and classic masterpieces like Little Women and Great Expectations. I use this rating very sparingly.)
4/5 - a very good book, loved it and would highly recommended it
3/5 - a good book, there were elements I did not like, but overall I liked it and would recommended it
2/5 - I read it (or most of it), but I didn't enjoy it and wouldn't recommend it
1/5 - major problems, didn't even finish the book (NOTE that I don't typically rate books that I don't finish so I have yet to use this rating)
4/5 - a very good book, loved it and would highly recommended it
3/5 - a good book, there were elements I did not like, but overall I liked it and would recommended it
2/5 - I read it (or most of it), but I didn't enjoy it and wouldn't recommend it
1/5 - major problems, didn't even finish the book (NOTE that I don't typically rate books that I don't finish so I have yet to use this rating)
I wish that Amazon used a 10-point scale. Often I would rather rate a book a 7 or a 9 out of 10, but have to choose the lesser 3 or 4 out of 5 possible stars.