I think one of the biggest reasons that books aren't rated (besides there being so damn many of them) is that most people who are reading a book will stop reading if it is something they are uncomfortable with. For example, if any of you have read the "Clan of the cave bear" series, you'll know that it portrays sex in several places. My wife read the series when she was a pre-teen (with parental permission) and being uninterested in that sort of thing, she just skipped over those parts.
Reading takes work as opposed to a movie, so the majority of people that would actually encounter those books by accident are those people mature enough to stop reading if they were uncomfortable with it.
Another thing to consider, if you've never experienced some of this stuff, how would you picture it in your head? Movies portray the imagery that the director has come up with for a certain scene, essentially removing the need to know what the hell is going on yourself, since the director has done it for you. All you gotta do is watch. A book, if you read words (even if you know what they mean) doesn't mean you can understand what is going on. If some guy is describing a murder or rape or something in all its bloody and horrific details, if you don't know what that looks like, chances are you won't understand anyway.
edit: ps, I'm also an avid reader. We have two shelves backed with enough books to fit on probably 5 shelves and I've read every one...wel saving a few I picked up used and still haven't read. Anyway, I go through kicks of reading fits where I'll read a few dozen books in a couple weeks, then don't read for awhile, same as video games =)