The line ‘as you know’ should never, ever occur on the first page of a book. Or even any page of a book. It’s always used as a rather contrived way of providing setting or back-story:

“Well, Prince Bob, as you know, your parents are the king and queen of this country, and as you know, they gave birth to you precisely seventeen years ago, because as you know, today is your birthday, etc.”

I don’t mind so much the strategy of introducing an outsider into a well-established world so that someone can explain to them everything the reader needs to know. It may be a rather obvious device, but at least it makes sense. People in normal conversation never say ‘as you know’ any more than they ‘swear under their breath’. I think it’s another case of people writing what they’ve read other people write without thinking it through properly.