I rate books like I rate the ladies.
Too many people hear someone called a "five" and think she must be ugly. In fact, a "five" is average. It's the average woman you see walking down the street everyday. You probably wouldn't chase after a "five" in a bar and buy her drinks, but if the "five" sat next to you and bought you a drink, I don't think you'd get up and leave out of principle. A lot of the books you read are "fives". They're not bad; they're just not good, either. If your house burnt down, chances are you wouldn't shout out "My copy of... was in there!" Chances are you wouldn't even remember it was in there until months later.
"Sixes" and "sevens" are generally what you end up settling with for the night. They're cute, but not beautiful. They have personality, but not to the point that you'll let them go on, and on, and on. Ten years ago, they were your stand-by until you met someone better. Ten years from now, you'll be thinking how nice it would be to land one of them. Your book-equivalents are kind of the same way. You're not excited about them, but you enjoy them nonetheless. You find yourself thinking of a half-dozen or more ways they could be better, but, again, you're still enjoying yourself.
An "eight", in my opinion, is a woman that could make money off her looks. Based on where she ranked, decimal-wise, between 8 and 9, you'd be thrilled if she gave you the time of day. Similarly, an "eight" for a book is (or should be?) good enough for a writer to not worry about choosing writing as his or her vocation.
A "nine" in my eyes is the lady that not only makes money off her looks, she makes considerable amounts of money. If your book is a "nine", it's the equivalent of your Victoria's Secret catalogue models. It's wanted, adored, and flipped through repeatedly. It's in the paper, it makes news when it's bad (behavior-wise, not in terms of quality). People still want it, nonetheless.
A "ten" is Helen of Troy. It doesn't exist; it's a fable--an ideal. The Iliads and the Odysseys of the literary world are "tens". But you know what? Read the Iliad or the Odyssey--I dare you. They're not as good as you imagine. They have captured the imagination, though, and have remained in our consciousness, like Helen, for over twenty five centuries. Centuries. If you're not a statue of a blind man, you haven't written the literary equivalent to the face that launched a thousand ships. And if you are, then guess what: your shit sometimes gets repetitive, and your character conflicts are often telegraphed. So there.
"Fours" and below. Why? WHY? Unless you're completely shit-faced, you shouldn't be messing with one. When you wake up to a four, you should run. If it's a book, throw it away. Don't waste time. Books, like women, are there as a balm to man's soul, to bring joy (not to mention pleasure) in life. If a book hurts to read... move on.