The letters finding their way to the Bureau of Labor Statistics (BLS) are testament to the growing restlessness of American workers:

"I'm dissatisfied with my job," writes one variant of an increasingly popular refrain being heard by Washington BLS researchers. "I've heard computers are a great field. What can you tell me?"

A good deal. This is research central for the Occupational Outlook Handbook, reputedly the most widely used career-reference source in the country. And lately, BLS author-economists see their biennial analyses of 200 jobs -- descriptions, pay, training and outlook contained in a phonebook-size compilation -- going beyond the traditional high-school and college student population.

"I suspect it's being used more and more by career changers or adults interested in upgrading their jobs," says Michael Pilot, manager of the Occupational Outlook Program, which has recently published its 1984-85 edition.

"There are more adults in the education system now. Consequently we can expect more career counseling being done with older workers, and the handbook is among the tools used."

According to the National Center for Education Statistics, the proportion of adults aged 30 and over enrolled in college courses has risen steadily in recent years: from 16 percent in 1972, to 23 percent in 1982, to a projected 28 percent in 1987. The primary motivation for these older students, center surveys show, is jobs.

For prospective job-changers considering a belated jump, for example, onto the computer bandwagon, Handbook authors offer this caution:

Accompanying the field's rapid growth is a rise in educational requirements and competition. Programmer applicants with less than a four-year college degree in computer science, they warn, could face difficulty in finding jobs.

The authors issue a similar caveat on health-field jobs, another area expected to grow rapidly. Should efforts to hold back rising health costs succeed, related job growth could be curtailed.

Among other handbook predictions: improved job outlook for elementary school teachers and construction workers, and stiffer competition for choice medical residency posts (because of the increasing numbers of medical school graduates).

"We've done a reasonably good job of determining the direction of changes since the mid-'60s ," says BLS economist Thomas Nardone, but the Handbook's projections were off, he concedes, in one area: "We consistently underestimated the labor force participation of women in the 1960s and '70s. We didn't gauge the full size of it."

Nardone says the Handbook offers a starting point for information about an unfamiliar field. "In terms of actually finding a job -- if someone's unemployed or laid off . . . that's not what the Handbook is designed for."

The Handbook (BLS bulletin 2205) is available in softcover for $8.50 from the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. It also is a standard reference in many high-school and college counseling offices and public libraries.

And now the bad news. Workers getting employer support for retraining now must pay income and Social Security taxes on the value of that aid, because Congress did not extend Employee Education Assistance provisions of the U.S. tax code.

In one case currently being submitted to Congress, an employe of the Caterpillar Co.'s Mentor, Ohio, plant was promised full employer reimbursement for a $900 data-processing course she could not afford on her own after the truck-making plant closed in late 1983. She finished the course in January 1984, a month after the tax exemption provision expired. Instead of $900, she received $589.54 after federal, state and city taxes were withheld. The balance had to come from her pocket.

A spokesperson for the American Society for Training and Development (ASTD), which provided the example, blamed Congress' action on "a misunderstanding that it would affect only upper-level employes." The ASTD is heading a coalition of business and labor groups calling for congressional reconsideration.

Now here's a howdy-do: How do you address a business letter when you don't know the gender of the addressee? Drop the salutation, says the Administrative Management Society. No more "Dear -- -- -- ." Instead, the society proposes a subject line in capital letters, such as THANK YOU FOR YOUR ORDER.

For women thinking of a military career, a new book promises to tell what recruiters won't. Everywoman's Guide to Military Service (Liberty Publishing, Cockeysville, Md., $7.95) examines the options -- along with the drawbacks -- for women in the U.S. armed services. Military recruiters fielded 1 million queries from women last year, according to Defense Department figures. In the Washington area, the book is available through Waldenbooks or the Pentagon bookstore.

Tupperware and cocktails?

Wait until the girls at the office get a load of this one. Tonight's Strangest Tupperware Party award goes hands down to the one for about 300 businesswomen -- "after work, of course" -- at the Brick Street Saloon at 1122 18th St. NW.

"I'm getting the most insane phone calls from people who think I've lost it," laughs PR executive Linda Roth, hosting the party with two associates in her firm. She swears it's on the level for harried working women like her who "get tired of saving cole slaw containers from Safeway . . ."

Who knows? If the idea catches on, there could be Tupperware beer mugs, tumblers, wine glasses . . . And your neighborhood bar may never be the same.