Should books be rated like movies?

Should books carry ratings like G, PG, R, and X, and should libraries and book stores have restrictions regarding what age you need to be to buy a book with a certain rating?

How are movies rated? Who does it? If you thnk books should be rated, who would do it?

Regarding your children did you let them read anything they wanted to, even if the material was sexually advanced for their age or very scary or violent?

Parents would be able to buy any book for their children but the child could not take it from the library or buy it themselves if they were too young for the rating.