Bob, you mention that almost every book says quit while you are ahead. I agree with your statement, but you must take a few things into consideration. Who the book is written for, the quality of the information in the book and the context in which the advice is given.
If the book is not about advantage play and does not tout a winning system but instead is a kind of fun book with pointers on how an amatuer can get more enjoyment out of his gaming dollars, then telling him to quit while ahead, fits as decent advice in this kind of book, in my opinion. Of course they never explain to him that he might land up with many short winning sessions which are greatly destroyed by several marathon losing sessions.
Then you have those many "winning system" books in your local bookstore that are full of poor information but sell because they are touting how to win with mathematically unsound advice and progressions. Here, the statement "quit while you are ahead" is a hedge used by the author so he can lie and say, my system works.
ihate17