Books predate society's desire for rating systems. They get a free pass because they're deeply ground into our mind as inheriently good tools of the advancement of civilization and yadda yadda.
I mean, what makes reading a Stephanie Plum novel inherently better than playing Mass Effect, other than that society says "reading is good," towards books and that "video games rot peoples' minds and their time would be better spent doing something more productive"?
Also, a lot of new media now is being appraised for what it is being used for now instead of what it can be used for. Film in the general public is primarily a form of entertainment, so ratings are often customary. Video games especially get a bad rap because its only within the past 20 years or so that the medium is able to have significant impact on the individual and society beyond a very few titles made with a combination of genius, hard work and lots of luck.
At the point that we'd ever consider organizing a rating system for books, they've been used as a format to address hundreds of millions of topics, have offerings for almost all age groups and peoples, and have changed the world many, many times.
Now with music, that's a bit different. Its more like a part of the human condition than a format for communication devised by humanity, and would perhaps be easier to look at ratings for music like how we have taboo in our language. Only certain things get the priveledge of being socially accepted.