I go to common sense media when I want to know what is in a movie I am about to watch with a movie. The language, sex, drugs and violence.
Is their a book equivalent? Common sense media seems pretty sparsely populated when it comes to books and their recommended age ratings.
Good question; no idea but registering for comment tracking if someone comes up with a site. ;)
A couple of possibilities
https://sites.google.com/site/parentalbookreviews/
http://www.compassbookratings.com/