In an age that applauds "letting it all hang out," says financial writer Thelma Kandel, "one topic is still taboo.

"People will tell you about their affairs, their illegitimate children and their favorite sexual positions. But they won't tell you what they earn. Even among close, close friends, talking about salary is Just Not Done."

Much of this reluctance "is due to embarrassment . . . or not wanting to brag." But keeping salaries hush-hush, she notes, is also in the employer's best interest.

"Management wants to keep employes in the dark about salaries," contends Kandel. "If you find out that a co-worker with your same seniority and experience is making more than you, you'll want a raise. But if you don't know what other people in your office or your field are making, you'll probably accept less than what you're really worth."

Salary secrecy is a particular problem for women, says Kandel, since "young girls are taught that it's not polite to discuss money." This earnings ignorance is one reason why, she says, "women still earn less than men.

"Despite great strides made by working women over the past decade, there has been no change in the relative earnings of men and women. We all know that women are underpaid. But when you see the figures in black and white, it's very depressing."

For example, she notes:

* Less than 1 percent of full-time working women earn $25,000 and over annually, compared with 12 percent of men.

* Two-thirds of women working full time earn less than $10,000 a year.

* Male high-school drop-outs earn, on the average, $1,600 more than female college graduates.

* The median annual salary is $19,433 for male college graduates and $12,028 for female college graduates.

* One out of 10 female workers earns as much as males in similar jobs.

* The average full-time employed woman earns just 59 cents for every dollar earned by men.

* Women in technical and professional jobs earn less than 71 percent of men's median weekly salaries.

* Although a woman chemist is paid a starting salary comparable to a man's, in 40 years she'll be earning almost $19,000 less than the same man.

These are among the grim facts Kandel sets forth in What Women Earn (Simon & Schuster, 160 pages, $6.95). The book examines the wage gap between men and women and peeks at the paychecks of women in more than 60 occupations -- from accountant to waitress.

"The whole point of the book," says the 49-year-old New Yorker, "is to give women a picture of their true market value so they can start earning what they're worth."

Kandel became interested in salaries several years ago, when writing an article about women in banking. "I heard the same thing over and over. Women kept saying they were making a lot less than the man who had the job before them.

"The old excuse for paying men more was that they needed the money to support a wife and kids. That's baloney . . . salaries are never determined by need. But even though the majority of women work because they need the money, their salaries are kept down by the myth that they are just working to buy 'little extras.' "

Other reasons for women's low earnings, she says, are "lack of unionization" and "being channeled into lowly-paid, female-intensive jobs. About 75 percent of working women are employed in undervalued occupations -- clerical and service jobs, nursing and health care, teaching, social work, retail sales and factory work."

Not surprisingly, she says "the best opportunities for bigger earnings are found in male-dominated fields like engineering, accounting and computer technology. Many of the occupations that pay the highest salaries in the United States employ the fewest females -- pilots (1.4 percent women), engineers (2.8 percent) and insurance actuaries (6 percent)."

One positive note for Washington women is that the wage gap between men and women is smaller in the District than anywhere else in the country. Washington women earn 78.4 percent of what men earn, followed by New Jersey with 66.1 percent and Vermont with 65.4 percent. Louisiana is last . . . women there earn 49.8 cents for every dollar earned by men.

Kandel credits the relatively high earnings of Washington women to the large proportion of people employed by the federal government. "But government is not such a great place for women to work," she says. "Women's positions are mostly in the lower of the 18 grades where pay and responsibility are less."

Women who want to improve their earning power, she says, "should look towards a field, like data processing, where there's explosive growth. There's not a large pool of men with experience there, so women have a good shot at becoming managers."

Another earnings booster is "tranferring your skills to an area that employs more men. Say you're great at sales. Get out from behind the counter at Woolworth's and start selling items with a higher price tag, where there's a commission, like major appliances. Selling equipment to business is a terrific field.

"Or specialize within your field. Don't just be a lawyer, be an entertainment lawyer -- their earnings are supposedly the highest. A secretary might make $18,000, but a biligual, medical, legal or executive secretary can make $40,000."

To make sure you're getting what you're worth, she advises asking friends in the same field what the salary range is for people doing your job at their firm.

"If it's more you're making, you may want to get another job offer," she says. "Then go into your boss and say, 'I'd like very much to stay here, but I have another offer at $5,000 more. I'd like to know what my future is here.'

"You're putting yourself on the line. You could be told goodbye. But that's the way to negotiate your way up. Bosses respect the fact that you're not selling yourself short."

While Kandel admits that "salary shouldn't be the only factor you consider in choosing or changing jobs . . . it's a pretty important consideration. Because even if you like your work, if you feel you're underpaid it can erode your self-worth, and, eventually, your job satisfaction."