All books are rated on a 1-10 Scale, with 10 the highest possible score. Here's what the numbers mean:
1-4.5 - Various degrees of badness. Not recommended (unless you're in a masochistic mood).
1 - One of the worst books I've read EVER.
2 - One of the worst books I've read this year.
3 - One of the worst books I've read this month.
4 - Hmm. You should probably stay away from it, but it won't kill you or anything.
5 - Indifferent. If it's one of your favorite genres, go for it. Otherwise, don't waste your time.
5.5-10 - Various degrees of goodness. Recommended.
6 - Generally recommended for transitory diversion.
7 - Highly recommended as a good, thought-provoking read.
8 - One of the best books I've read this month.
9 - One of the best books I've read this year.
10 - One of the best books I've read EVER.
I base my ratings on three criteria, weighted approximately equal:
1) INTENT - Does the book fulfill its purpose and match the author's ambitions?
2) TECHNICAL MERIT - How well-written is the book? Is it stylistically remarkable?
3) APPEAL - Quite simply, did I personally enjoy reading it?