Her death was confirmed by Mary M. Mitchell, a longtime friend and collaborator.

At 35 Ms. Baldrige, known as Tish, left her job as public relations director for Tiffany & Company to help out a friend and fellow Vassar alumna, the former Jacqueline Bouvier, becoming, in essence, the social secretary of the Kennedy White House as it emerged as a center of culture, art, youthful elegance and sparkling state dinners.

Ms. Baldrige left the White House in June 1963, less than six months before President John F. Kennedy's assassination, to work for the Merchandise Mart, a Kennedy family business enterprise in Chicago. She went on to found her own public relations and marketing business.

In the 1970s she established herself as an authority on contemporary etiquette, writing a syndicated newspaper column on the subject and updating "Amy Vanderbilt's Complete Book of Etiquette" in 1978, less than four years after Ms. Vanderbilt's death. Ms. Baldrige's face soon appeared on the cover of Time magazine, which hailed her as the nation's social arbiter.

After that, her own name was enough to attract readers, and in 1985 she published "Letitia Baldrige's Complete Guide to Executive Manners," which dealt with behavior in the workplace and outside it. In that book, she declared it acceptable to cut salad with a knife. She recommended that whoever reaches the door first — either man or woman — open it. And she suggested infrequent shampooing when staying on a yacht, to be considerate about conserving water.

Ms. Baldrige, who stood 6 feet 1 inch tall and became known for her elegant silver hair, long contended that the heart of all etiquette was consideration for other people, rather than a rigid set of rules.

"There are major C.E.O.'s who do not know how to hold a knife and fork properly, but I don't worry about that as much as the lack of kindness," she told The New York Times in 1992. "There are two generations of people who have not learned how important it is to take time to say, 'I'm sorry' and 'please' and 'thank you' and how people must relate to one another."

In addition to her all-purpose etiquette guides, she narrowed her focus in books about weddings, social lives, job success and child-rearing. Even when she went far afield of her specialty, as with "Public Affairs, Private Relations" (1991), a novel about romance and class differences in Washington, she threw in comments about manners.

She wrote at least three books that capitalized on her brief, shining White House career: "In the Kennedy Style: Magical Evenings in the Kennedy White House" (1998, with René Verdon); "A Lady, First: My Life in the Kennedy White House and the American Embassies of Paris and Rome" (2001); and "The Kennedy Mystique" (2006, with four co-authors). Those books' revelations tended toward menus, recipes and minor shockers, like Mrs. Kennedy's habit of referring to Helen Thomas and another newswoman as "the harpies."

In a 1964 oral history interview for the John F. Kennedy Presidential Library, she remembered the Kennedys as perfectionists and the president as an amazing manager.

"He was like a wonderful department store manager who goes through the store and knows everybody's name and knows how all the departments work and knows how to wrap packages better than the wrappers in the wrapping department," she said.

Letitia Baldrige was born on Feb. 9, 1926, in Miami and grew up in Omaha, the youngest child of Howard Malcolm Baldrige, a Republican state legislator who became a United States congressman in 1930, and the former Regina Connell. (Their son Malcolm was secretary of commerce in the Reagan administration.)

Growing up with two older brothers helped make her tough, Ms. Baldrige said. Speaking to her hometown newspaper, The Omaha World-Herald, in 1997, she recalled the time her brother Robert had swung his new baseball bat, a holiday gift, too close to her. "I was knocked unconscious for three hours," she said. "My brothers called it the best Christmas so far."

Like her future employer Mrs. Kennedy, Letitia attended Miss Porter's School in Farmington, Conn., and received a bachelor's degree from Vassar College. She did graduate work at the University of Geneva in Switzerland but still found that she had to learn secretarial skills to find a good State Department job.

Beginning in the late 1940s, she worked in Paris as social secretary to David Bruce, the United States ambassador to France, and his wife, Evengeline; then in Rome as assistant to Clare Boothe Luce, at that time the ambassador to Italy. On that first job she made a major faux pas by unknowingly seating a Frenchman next to his wife's lover at a dinner party. As a result, she often said, she learned the value of heartfelt, repeated apologies.

When she returned to the United States, she went to work for Walter Hoving, the chairman of Tiffany & Company. Her first book was "Roman Candle" (1956), a memoir about her European adventures, which one critic, Elizabeth Janeway, accused of managing "to invest Rome with as much color and atmosphere as if it were her native Omaha." Her last book was "Taste: Acquiring What Money Can't Buy" (2007).

Most of Ms. Baldrige's career was spent as an entrepreneur, as head of her own businesses in Chicago, New York and Washington, where she had a home at the time of her death.

Yet she continued to be identified with her White House days. "That's all right," she told The Times in 1998. "It was a moment in history, and to be part of it is incredible."

Ms. Baldrige married Robert Hollensteiner, a real estate developer, the year she left the White House. He survives her, along with their daughter, Clare Smyth; their son, Malcolm Baldrige Hollensteiner; and seven grandchildren.

Family, Ms. Baldrige believed, was where the patterns for manners, humanity and true civilization were set, and the American family was failing to do its job.

"We are not passing values on to our children," she told The Toronto Star in 1999. "We are not sitting down at the dinner table talking about the tiny things that add up to caring human beings. Jackie learned from her mom, who had beautiful manners."