Any book club that sells books, that I have looked into, has *sexually explicit, foul language* or some such thing, on books that have those things in them. Also, Amazon encourages readers to talk about and discuss the books that they have listed, so perhaps you could browse those to see if anyone mentions that kind of thing.
My problem with the rating system for books is much like Pengwenn's. I don't want an author with a brilliant idea, or rich and wonderful story to tell, to be scared off, or encouraged to put more in, by what rating they will give the book later. If there is an adult story that needs no foul language, or sex, or whatever to be told, the author shouldn't be encouraged to add that just so the book will reach the target audience. And by that same vein, I don't think an author should be afraid to be true to the story for the same reason.
Unfortunately, kids are growing up faster than we would like sometimes, and it is our job as parents to know what they read and to help them make those choices. The same goes for movies, and even if something has a higher rating, or a low one, I almost always screen before I let my children watch it. Just in case, you know what I mean? I don't rely on the government, or the movie rating system, to choose what my children watch on tv, or their movie watching. I decide for myself. And if there is a PG movie that I am alright with my five year old seeing, then he can watch it, or if there is a rated G movie that I don't think he would like, or might scare him in some parts, he doesn't watch that. The long and the short of it is, parents need to be proactive in their kids lives, and warnings on books would be one more step to taking all of the "guess work" out of choosing things for your kids, supposedly.
Basically, movies, games, music, and television are all rated now, so that we as parents don't have to worry about what our kids are watching, listening to, or playing, without having to screen them ourselves. For me, that doesn't really work. I very rarely let my kids watch something that is above their level, but only because I have watched it myself and seen the kind of show it is. I also have kept them from watching some at their level, also because I have watched it and am not alright with it for my younger son, or would encourage my older son to be more aggressive toward his younger brother. He is easily influenced by television and movies, so I am careful. We, as parents or responsible adults, have to be responsible for what our children are doing, reading, listening to, and seeing. We can't blame censorship, or the lack thereof, for the choices that we make with our kids.
Please, don't misunderstand, I know that this group is a very proactive group of parents. I meant that in a general term, not one that applied here. And I know, that as a community here, many of us come to ask advice on books and movies for our children. Having a place like this, or even a site that you can go to and look up a book to check the content, is more than enough for proactive parents. And I don't think that you could count on a non-proactive parent to even know what their child is reading, much less the rating on the book. I am sorry that sounds harsh, but if my sister was ever asked what kind of books her daughter read, she would be hard pressed to answer you. My niece is 15 and pregnant. I don't wonder why, I know my sister. And I don't think that putting ratings on books is going to stop that kind of parenting, or lack of it. And it wasn't reading books that got my niece pregnant either.