We Need Ratings for Books

Guys, I wish books had ratings like movies did.

Now, I can see someone commenting down below something along the lines of,

"But Graayyyyy, they do have ratings!! You have jfict books written for twelve and below, YA written for teens, and NA written for a more college-aged audience, and then just Adult fiction written for, well, either advanced readers or older readerssssssssssss!!!!"

But the thing is, one can go to blockbuster (okay, maybe not anymore, but I have to bring back that nostalgia *sighs* the good old days that weren't actually that good but I remember them in a golden filter for some reason),

ANYWAYS, people could go to a certain section of the genre of the type of movie they wanted to see, but every movie in that genre also has a  s e p a r a t e  rating.

For example, if I go to the horror section and pick up Signs, I can turn it to the back and see that it is rated PG-13 for some more intense moments.

Then if I pick up let's say Along Came a Spider I could easily find out that it is rated R for some violence and language. 

For some reason, we don't have this system when it comes to books??? 

I could pick up a romance book from the YA section and it could be innocent and cutesy, but I could pick up another book from the same section and the characters could sleep together in graphic detail on the second page. 

The only warning we really get about the content in any young adult book is the cover + blurb, and both of those can be deceiving. 

I have four younger sisters who are all (obviously) going to advance in reading soon, two of them are already at a high school level, but of course, we can't just let them read whatever young adult book they want because so many young adult books have language and sexual content. 

I get that everyone has different standards, but I wish readers got more of a warning so those with different morals, triggers, and beliefs could avoid upsetting content. 

Hollywood can do it, why can't our publishing houses? Why can't our indie authors?