Who would rate the books, that's the crux.
The problem I see is the same as with the motion picture rating situation. The ratings eventually have tremendous effect on sales, and so the raters end up with a crazy amount of sway. In the movie industry a G rating will kill you in the box office as surely as an NC-17, no matter how good your movie is.
check out this article on the problems with movie ratings:
Debating the MPAA's mission