JIMMY CARTER's eight-country swing around the planet earth is only peanuts compared to what anthors do to promote their books. The 24,000-mile tour to sell the thing is part of the romance of getting it done, One-night motel stands, 7:30 am talk shows, soggy muffins, weak coffee and missing book shipments are the folkways of a literary career. Limping home is shreds to pick up the remnants of a shattered life is the hallmark of the writer who's got it made.

Except Deborah Szekely Mazzanti is out there, bucking the trend. This week - for the second half of her tour - she hits Portland, Seattle, Phoenix, Houston, Dallas, Tulsa, Cleveland, Atlanta, Palm Beach, Kansas City and Topeka, to push the sales of Secrets of the Golden Door (Morrow, $12.50), which came out last month. The first lap took her to New York, Washington, Boston, Minneapolis, Chicago, Denver, San Francisco and San Diego, where she lives.

Is she in shreds? "Not a bit. It's like a vacation." Do her publishers forget to send copies of her book? "Of course not. And if they do, I've just told them I'll take the day off."

The secret of survival on the road is not to visit a health spa before you leave. but to own one. In fact, Deborah Mazzanti owns two of them. And it helps to have plenty of help. Her Rancho La Puerta, which opened in Baja California in 1940, accommodates a hundred guests with a hundred staff members to keep them in line. The Golden Door, a lavish $3.5 million Japanese-style spread in Escondido, California, has a staff of 90 attending to the needs of 30 guests. "Forty per cent of the gross goes into payroll," Mrs. Mazzanti says. In addition, she runs a Golden Door cosmetic company, and is planning to market some of the vegetables she grows on the sweeping acres that surround The Golden Door.

So, although "writing a book is like going naked down the street - I'm a talker, not a writer," as she says, its promotion has been a triumphal passage among the alumnae and alumni of her spas. (Except for three days in Topeka, where she's on the board of the Menninger Foundation.) And since a week at The Golden Dorr runs to $1,250, plus 15 per cent in "gratuities," the alumnae run to the likes of Barbra Streisand, Kim Novak, Dorothy Kirsten, Roberta Flack, Barbara Howar and Jolie, Eva and Zsa Zsa Gabor. Men's Week "We gentle you and police you" for a "tension-relieving" week, the promotion folder says) draws people like William Holden. Bill Blass, Stanley Kramer and the late Aldous Huxley, who wrote about Deborah and her brother in one of his book. A member of the Hermes family comes there all the way from France (and entertains Deborah with eclat when she ventures abroad). "My guests are achievers," she says, "but they lose simplicity of goals." Her graduates are as loyal as the Princeton Class of 1912. They treasure the frayed jump rope Deborah sent them home with. They write touching reports of their changed habits, their mended ways.

In Washington, where she addressed a sell-out breakfast at the Tyson's Corner Bloomingdale's, appeared on two talk shows, was guest of honor at a dinner and spoke to an American Newspaper Women's Club Sunday brunch, old grads were close at hand. "My guests make the best shills," she said. At a Women's National Democratic Club lunch, she was introduced by Senator Alan Cranston of California, who waved a copy of her book at the crowd, read "extracts" from it and called her "a very, very remarkable woman - a wonderful constituent and wonderful friend" who also "got the biggest small business loan in S.B.A. history."

In both the book and the talks she gives, she markets the theories of her luxurious spa without its lavish decor - The Door without walls. In the rumpled setting of your own house, she's advocating lying in bed in the morning, "not really quite awake," counting your fingers and toes. ("Just wiggle and know you're home.") Next come two minutes of nude exercises in front of a full-length mirror, ("Decide to live in your body; not just be a passenger going through.") After that, at least one hour of movement a day - not just a languid stroll around the block, but "something huffy puffy. Of course it's no fun. Nobody in their right mind would like it; it's work."

Meals should be the "proper amount for your container size. Eat in ethnic restaurants where people are small and you'll lose weight . . . And envision food as yourself . Do you want to be a crisp apple? Or a crunchy carrot? Or a doughnut?"

Then to re-order your life, you keep a calendar for a month, marking in black "everything you did that you didn't like"; in blue "for duty: earning money, chores, family responsibilities, and the like"; red for health; green for "growth, your personal growth"; lavender "for everything that was joyous."

Meanwhile, too busy to follow her own regime, she herself would never be described as svelte. She is unconcerned that her hair is turning gray. When her publishers hinted at a facelift before the selling tour, they got turned down flat. "I don't think it's becoming, and besides I'm chicken." She is a vegetarian, but she eats meat when it's put before her and serves it her spas. "I hate fanaticiam," she says. "And besides, otherwise I'd go broke."