E10+

[A Black Falcon]

If you want ages, it's basically...

Ec: up to 6 (young kids' educational games)
E (G): 6+ (okay for anyone)
E10+ (PG): 10+ (some material not good for young kids)
T (PG-13): 13+ (because children are quite different at 13 from how they were at 10, they realized another category was needed... otherwise you say "okay, is this a high PG or a low R?" and aren't really quite satisfied with either.)
M ®: 17+ (some content restrictions, but not as many)
Ao (NC-17): 18+ (very few games in this category, and no major commercial titles.)

As for books, it's true that they aren't rated, but they are seperated -- children's books are put in one place, adult's books in another, in either a bookstore or a library. That serves for the most important purpose, seperating kids' from adults' stuff... but anyway, how would you content-rate a book? It seems like it'd be harder than movies or games... you could do it, I'm sure, but obviously no one has made an issue of it so it's not been tried. Just banning unpopular books alltogether.