In 1979 after nearly 20 years working in advertising and as a congressional aide, Andrew Harper, 44, of Fairfax Station knew he was ready for a career change when he realized he was happiest on vacation.

While contemplating what his "mid-life career change" should be, Harper weighed heavily the fact that the two things he loved best were writing and traveling. "But I didn't want to end up buying a travel agency like all my friends suggested." One reason: As a travel agent he'd have to deal with groups and "recommend the kind of large hotels that I disdain."

Instead, Harper got the idea of traveling around the world in search of the types of places he would like to stay in -- exclusive inns, manor houses and island retreats -- and then writing about them for a select, well-heeled audience. Which is exactly what Andrew Harper does today as editor and publisher of his own monthly newsletter, The Hideaway Report.

Newsletter publishing is one of the fastest growing and most profitable segments in the information industry, according to Howard Penn Hudson, president of the Newsletter Clearinghouse, a service organization for the industry, and editor of The Newsletter on Newsletters. And newsletters, he says, which now number about 100,000 in this country, "promise to continue their surge into the '80s."

"Newsletters are so much a part of your life," says Hudson, "that you may not realize how dependent you are on them for information."

Newsletters are also part of the trend toward specialized interests. "People seem to think they know exactly what they want to read about and don't want to be bothered with anything else," observes Hudson.

And the wild variety of subjects that people want to read! A glance through the Oxbridge Directory of Newsletters, which lists 13,000 newsletters published in the U.S. and Canada: Bill James Baseball Abstract, Cardiac Alert, Deltiology, Milkshed Memo, Sludge, Hog Call Fanletter and Spirit of St. Louis, to name a few.

"A newsletter starts with an idea, either for the publication itself or for the audience to be served," says Hudson, author of Publishing Newsletters (Charles Scribner's Sons, $19.95). The first documented newsletters, he says, were produced by German Count Philip Edward Fugger in the 16th century. The loose, handwritten sheets reported business news gathered by various agents in trade centers throughout Europe and overseas. Also discovered in the Fugger archives were news reports or letters sold to paying subscribers. "Here we have the basic characteristic of newsletters," says Hudson, "specialized information prepared for a subscription audience."

The first modern newsletter (1918) was the Whaley-Eaton Report. In 1923 Willard M. Kiplinger founded his famous Kiplinger Washington Letter and set the style that was to become the classic newsletter format: a typewritten report to businessmen in the form of a letter from an insider interpreting government activity.

Despite this obvious formula for success -- tell a man something he needs to know and he'll pay you for it -- newsletter publishing did not catch on until the '60s when "need-to-know" information became imperative for American business.

Frederick D. Goss, executive director of The Newsletter Association, the industry's trade association with over 800 members (publishing 2,000 newsletters and specialized information services), credits Lyndon Johnson for the newsletter boom that began with the Great Society.

Goss says that in order to keep up with the explosion in the Great Society's federal programs, public policy experts started typing and entrepreneurs started selling. Soon newsletters on labor, business, economics, environment and safety, law, taxation and every other public policy issue began appearing. Today Goss estimates there are approximately 4,000 commercial newsletters, a third of them published in Washington. "They used to say at The Bureau of National Affairs, if it's a new law, it was a new newsletter."

Certainly, The Bureau of National Affairs, Washington's largest publisher of "specialized information services" with 70 daily, weekly, monthly and bimonthly reports (such as U.S. Law Week and Daily Report for Executives) has over the last 20 years transformed the science of "need to know" into an industry. BNA employs nearly 1,200 persons and run as high as $3,000 annually for daily reports.

America's newsletter boom also continues to be attractive to individual entrepreneurs for two reasons, according to Goss, author of Success in Newsletter Publishing (The Newsletter Association, $37.50): relatively low start-up costs and computerized ability to target a potential market of subscribers. But of the estimated 1,000 new newsletters to be launched this year, Goss warns, "more than 50 percent will fail."

Neophyte newsletter entrepreneurs, he stresses, should assess how specialized their information is, check their competition, target realistically their market of subscribers and then test their idea with a direct-mail marketing campaign before ever publishing the first issue. He and other experts recommend a minimum of $10,000 for start-up costs, although some successful beginners have done it for less.

"Successfully publishing a commercial newsletter is a complex business," says Goss. "People come into it thinking, 'I'll get a list of people, send them a sample copy and an order card and the subscription checks will come rolling in.' " Stop right there, he advises. "It's just not that easy."

Easy or not, what makes newsletter publishing intriguing for would-be entrepreneurs is the success of such newsletters as The Old House Journal, Chocolate News and The Hideaway Report. All three began with little more than intuition and moderate investments by their owners. The results have been profits, pleasure and the experts' puzzlement.

In 1973 Clem Labine, then in the thick of restoring his Victorian brownstone in Brooklyn, N.Y., knew there had to be other homeowners across the country struggling through the same trial-and-error techniques of restoring an old house. Labine also thought these people would pay for friendly, helpful advice. He was right. Today the $18 do-it-yourself monthly has 85,000 subscribers, grosses over a million dollars annually and since January has moved out of the newsletter category and into the magazine market with a slick color cover, limited paid advertising and newsstand distribution. According to editor Patricia Poole, the Old House Journal incarnation as a magazine has been "extremely successful."

Newsletter experts can't explain the hit of New Yorker Milton Zelman's Chocolate News launched in 1980. This publishing gem -- a chocolate-scented bimonthly featuring recipes, shopping tips and interviews with celebrity chocoholics -- attracted 23,000 subscribers at the very low price of $9.95 a year. Recently Zelman sold the newsletter to another publishing firm that has raised its subscription price to $12.95.

Andrew Harper's success with The Hideaway Report also gives inspiration to all newsletter entrepreneurs because just about everything he did when he was starting his newsletter the experts said couldn't be done. "It's amazing," he says, "what the experts say you can't do."

Unlike most newsletter publishers, Harper did not use direct mail to target his subscribers, but a discriminating space advertising campaign in such publications as The Wall Street Journal and The New Yorker (one reason his start-up costs were about $18,000). Today The Hideaway Report grosses nearly $1 million annually with a limited subscription of 15,000 readers who pay $65 for his monthly newsletter. Harper's subscribers are so keen on his publication, he says they asked him not to go mass market and let everyone else in on their secret vacation hideaways.

"I wrote everyone a letter and said 'okay, but the price will have to go up (from $45 to 65)." No problem, the majority replied. The result is that Harper's exclusive newsletter enjoys an 80 percent renewal rate with new subscribers waiting for entry onto his list.

Despite his success, Harper does not recommend that novice newsletter entrepreneurs blindly follow his example. "I think it is important to listen to advice on how to put together your newsletter and how to market it. Certainly [the newsletter industry resources] provide valuable information. But in the end you really have to go on gut instinct."