I don't think books should have a rating, no. But there is one thing that might warrant a rating, or at least a list of items to consider.
Excessiveness
What I mean by that is this. Take a fantasy fiction romance book. Most include a deep plot, revenge, betrayal, adult situations, and possibly an act of "invasions of personal space"

This book is geared towards the adult market, and even though these are in the book, the book should not be rated as they are not excessive.
On the other hand, take Beauty's Release series, another book geared to the adult market, also a fantasy fiction romance, yet this one is full of adult situations and many acts of personal invasion. This excess would call for it to be rated because even though the book jacket may say what the book is about, the average adult reader who would be interested would most likely be disgusted by so much it.
Or perhaps, not a rating so much as a caution label.