THE PUBLIC sport or ruining people's lives by discovering their diaries or introducing their love letters into court is gone. Few people write such things any more, and as it has become the custom to deliver such information the custom to deliver such information about oneself orally and voluntarily at social events, one could easily suppose that there are no written personal secrets in existence.

This is not the case. The most fastidious people still keep recording information that would, if discovered, cut them off from decent society. Miss Manners is referring, of course, to the party book in which one keeps the names of one's friends under the classifications one has given them as guests.

Such a record is as useful as it is dangerous. In it may be noted guests' violent allergies, to food or to specific people, but the heat of it is a system that enables one to put together an interesting party.

At the risk of ostracism, Miss Manners will explain her system, not only to show off how magnificently organized she is, but to illustrate what makes a good combination of guests.

You may have heard of "A" lists and "B" lists. All experienced hosts classify their friends this way, but only the clumsy ones allow their guests to know, from looking about a room, which they are on. (A gentleman of Miss Nanners' acquaintance has the misforturn to live next door to a good friend who frequently entertains on her patio and has, through long, careful window observation, discovered that, much as he enjoys the parties to which she invites him they are not her first-rank ones. He has chosen the wise course of treating this as inadmissible evidence in their friendship.)

The A list should consist of what a lady of Miss Manner's acquaintance calls "sparkles." These are people who through their private status or through their talented efforts, can "make" a party. The greatest artist of the day could fill this position silently, for instance, or a nonentity could do it with consistently brilliant conversation.

The B list, like the ideal middle class, should consist of solid citizens with a strong sense of duty. The duty is to listen to the sparklies and to be able to carry a reasonable amount of good conversation.

Then there is a C list which, like poverty, one is always trying to eliminate but can't. These are the social obligations -- incurred through sloppy acceptance of their hospitality, ancient friendship from which the interest has disappeared, or the pleas of mutual friends -- who do not earn their dinner.

Miss Manners then cross-files these lists by occupation and level of achievement. One's friends tend to be from a limited number of fields. The question of achievement level is not snobbery, but the fact that people who are striving for advancement -- a category that naturally includes almost all young people and even some poeple who ought to be enjoying their high status but can't stop striving -- rarely give themselves over wholeheartedly to theoretical and disinterested conversation.

In the well-planned dinner for 10, there should be (taking out the host and hostess): two sparklies from different fields, four solid listeners and contributors from assorted professions, one charity case and one mystery guest whose classification will not be clear until after being auditioned at this dinner. Laurel resters should, on the while, be kept separate from ladder climbers, unless the latter show extraordinary talent as listeners. All sparklies make as dull a party as all audience. Unless occupations are mixed, you will hear nothing but shop talk.

Since a good party is such a mix, your best friends will never know what you consider them. Provided, of course, you keep your jaw and your desk locked. MISS MANNERS RESPONDS

Q. I've been engaged for three months and I've got a little problem that maybe you could solve. My mother divorced my father 14 years ago, and remarried. I came to love my stepfather very much, and I never really considered my stepdad as not being my real father, since he has raised me since I was 5 years old. I never really saw my real dad, and that never bothered me, because I was quite content with my stepdad.

My stepfather is even going to walk me down the aisle, because I asked him, and I wouldn't want anyone else to do it. But how do I go about filling out the invitations? I want my stepdad's name down and my mother says it can't be done that way. My real father and stepmother are invited to the wedding also.

A. Having been right once, in marrying such a nice man, your mother cannot expect to always be right. In this case, you are -- by all standards of kindnss as well as strictest etiquette. The correct wording is: Mr. and Mrs. Horace Drew Contentment request the honor of your presence at the marriage of their daughter Pamela Jean Larue . . .

Q. When a multi-story sandwich is served, such as a club sandwich, and it is held together with frilled toothpicks, what do you do with the toothpicks?

A. You take them out of the sandwich. What did you think -- you eat them?

Q. As a person who performs and who has performed "services" -- secretarial, clerical and so forth, I am much too full of bitterness and resentment against those people (probably people like you I'll wager) who enjoy the idea of a "lower class" of workers to whom they have the right to be rude, to be sympathetic to your view that "people in the service jobs seek compensation by behaving with inefficienty and surliness to those whom they are supposed to serve."

Yes, I have been the victim of some surly people in service jobs -- clerks who stand there watching me wait at the counter and don't come over to help me, and so forth. But I have also noticed how those who are being served -- doctors, lawyers, professors, professional people in the upward brackets, in particular -- show rudeness toward their "inferiors" as your snobbish expression puts it. They really believe that those lower on the pay scale do not deserve to be treated with dignity and respect.

I am a college graduate who worked service jobs while in college and I have always been a lady. Therefore, I have never been anything but gracious to everyone, whether they be college professors or trash collectors. I have noticed, however, that the professional "upper class" people are notoriously lacking in any genuine good manners or graciousness, unless they are courting the attention of someone who can assist them in moving higher up the economic ladder. Now, lest I be guilty of generalizations, as you are -- I hasten to add not all people in the "upper classes" behave like that. I have found the utmost grace in many groups of people -- even doctors and lawyers, believe it or not. With that, Miss Manners, I leave you to your snobbery.

A. Ah, you have not heard the half of it. Miss Manners' snobbery is such that it follows the 19th century practice of classifying doctors, lawyers, artists, professors and businessmen in the serving classes.

In other words, all of us who work for a living are in a position of service when our work brings us in contact with the public. Your doctor and your lawyer, if they but knew it, are as obligated to be pleasant to you as your waitress and your automobile mechanic.

As this includes just about everybody in America, who, you may well ask, are the members of the upper classes, and are they exempt from polite behavior?

In a democracy, we all get a turn at this. You, as patient, client, customer, are being served, and should be served well, by working people. And what keeps it imperative that you treat them graciously is noblesse oblige , the sternest master of all.

One last generalization from your Miss Manners: There is no snobbery like that inspired by those who aspire to the lower classes.