One would think that today's more casual life styles would have rendered etiquette books obsolete. Instead, the openness itself seems to have raised some new and intriguing questions.

Consider, for example:

Should an unmarried couple who have just had a child send out birth announcements?

Yes, says at least one manners maven, Charlotte Ford, whose "Book of Modern Manners" (Simon and Schuster, 509 pages, $14.95) is just out. They are "especially thoughtful," she says, "because they let friends know what the baby's last name is."

If you are a child living with your divorced father, how do you introduce his live-in friend to, say, your teacher?

Advises Ford: A child need only say, "This is Karen." A fuller explanation "would be awkward for all concerned."

If drugs begin appearing at a party -- "it happens" -- and you're a non-user, what do you do?

Don't "make a scene," but you're probably going to feel uncomfortable if you stay, so just get up quietly and "go home." If someone lit up a marijuana cigarette at one of her Manhattan apartment parties, says Ford, she'd tell him "to put it out."

No host should "be pressured" into offering marijuana or other drugs "or to tolerate its use by guests."

How do you introduce a married woman who has kept her own name when she is with her husband?

This one's easy. Say simply, "This is Edna Klepper and her husband, Lance Loomer."

If you've invited an unmarried couple for an overnight visit, where do they sleep?

That's "clearly up to you, the host," writes Ford. To friends who say they are worried about what to tell their children, she offers: "Small children generally aren't interested in who is sleeping where and why. Teen-agers know what's going on anyway."

Put guests in a separate room if you're going to be uncomfortable. It's best, she says, "to clarify the arrangements with your guests before they arrive."

If you're inviting a gay friend for a weekend, does the lover get an invitation too?

Yes -- "apply the same rules to a gay family as you would to any family."

Should a working wife be expected to attend social functions involving her husband's work (or vice versa)?

Says Ford: "Today, busy wives who can't find the time to attend their husbands' business evenings may bow out with the valid excuse of having work or engagements of their own."

When an agent asked Ford, 39, to do the book five years ago she had to be convinced there was a need. At the time, "Honestly, I hadn't even read Amy Vanderbilt and Emily Post." But she realized there are situations in our lives today -- "that people used to be too embarrassed to even talk about" -- not covered in standard etiquette books.

Though strict formality in etiquette "seems to be disappearing rapidly," Ford believes "even in informal settings, etiquette is important." She wrote the book "to help answer the whys and hows of situations for which there are no longer exact standards of proper behavior."

For Ford, the essence of etiquette is "logical, sensible patterns of behavior we can all follow in order to make our relationships with others pleasant, courteous -- and gracious."

Once she threw a party for which the guest of honor showed up -- in blunt terms -- drunk. She "nurtured him through dinner; he ate nothing." While the other guests adjourned to another room, the man's wife "immediately came up and told me, 'I think I'll take him right home.'"

That, says Ford, "was the absolute right thing to do. She was going to sneak him out. Nobody else realized what was happening."

As the daughter of Henry Ford II and the former Anne McDonnell Ford, Charlotte Ford grew up with money, her father reportedly flew in 2 million Mississippi magnolia leaves to transform the Country Club of Detroit into a French chateau.) Some of that rarefied atmosphere creeps into her book.

Thus, you get advice on such things as writing a marriage contract alongside proper manners in your opera box.

Twice divorced -- at 24, she eloped to Mexico with Greek shipping magnate Stavros Niarchos, who is the father of her 13-year-old daughter, Elena -- she keeps a large Manhattan apartment, a home in Southhampton and a "small" condominium in Sun Valley.

Her name is attached to a popular line of women's sportswear for which she "gives ideas," but does not do the actual designing. "I like to give as much input as I can." On a trip here, she wore a 4-year-old mauve Charlotte Ford jersey (which she also wore for her book-cover photo).

Ford has relaxed her own entertaining style -- she gives fewer formal dinner parties -- but she likes her guests to arrive on time. "I'd much rather have my guests come at the time I invited them than sit and wait for them to straggle in fashionably late. It's not fair to the person doing the cooking." s

She usually schedules a 45-minute cocktail hour -- time enough for two drinks. "Too much alcohol dulls the palate." Tardy guests are served whatever course the others are eating.

And if they don't arrive until dessert? "They'd better take a big helping."