I found this a fun and funny read at the same time.
About 70% of this book is things you'll never do. And I mean never. It has detailed instructions on how to reply to someone who has just come out of a divorce (if you need a book to tell you what to say to your friend who just got divorced, then you really need help), how to eat an avocado or baked potatoes "the right way", and how to behave in embassy dinners. Also, the stuff on dating and "things to say" and "things not to say" I think are quite irrelevant, and ignores a large section of "dating" today. I mean, by all means, if you want to be a "nice guy" and just be constantly timid and take almost no initiatives of your own, then go for it, but if you want to be attractive to other people, then there's other books to read which basically instruct you to do almost the exact opposite of what she says.
There's about 30% of this which is useful: how to lay out a formal table or buffet, how to serve food, how to eat various tricky dishes (which help minimise making a mess), how to address various people (ministers, queens, princes, ambassadors etc) when writing to them, etc.
But then again, this is American-oriented, and it ignores for most part important European conventions, such as the position of the left hand (or right hand for left-handeds) when eating: in the book it shows that the left hand is under the table, but in Europe the savoir-faire dictates that the hand is clenched in a loose fist, and the fist rests on the side of the table (elbows as close to the body as possible). As with most good manner "rules", this serves to be practical more than anything else: if you need to reach out for anything on the table you have to make a smaller move than if you had your hand under the table (resting on your lap); also, it is an indication of good intentions, as you show you don't have anything to hide (in older times you could be loading your gun to shoot the person across you!); furthermore, it provides with a more balanced posture at the table, and helps prevent you from moving your head towards the plate, rather than moving the fork with the mouthful of food towards your mouth.
For most part it does not indicate the *why* of a particular convention, but just lays out the convention which must be taken without questioning. I think the best way to teach manners to anyone is to explain the (practical or aesthetic) reason behind a particular convention, because not only will it be easier to remember, but it will also make sense. If there is no particular reason for doing something, maybe it's time to stop doing it.