SOMETIMES defined as "the world's second-oldest profession," and sometimes practiced by snake-oil salesmen with the "loosest of ethical codes, press agentry is one of the livelier crafts of theater.

Such folk as Lillian Hellman, S.N. Behrman, Russell Crouse, John O'Hara and Robert Benchley were theatrical press agents in their leaner days, and time and evolved such euphemisms as "councillor on public relations" or "public relations council," the term a masterly practitioner, Edward L. Bernays, persuaded the phone companies to introduce into the Yellow Pages.

Their assignment in theater is to spread the word about their employer's attraction to boost business and prolong its run. This is achieved by supplying facts and bodies to the hungry media. Unless a production is talked about, written about, audiences will not appear.

Between media people and press agents the relationship is a supreme example of trust and suspicion, love and hate. It is vital that the agent be a complete ghost. Only exceptional employers fail to have fits when the agent's name is mentioned and their own ignored.

Thus, media people, relishing their apparent omniscience, and agents, as anxious to keep their names out of print as they are those of clients who have been nabbed for the more sordid derelictions, veil the liaison. But when agents rise in the manner of Hellman, Behrman, Crouse, O'Hara and Benchley, it is permissible to mention their abandoned dealings.

Such a one is Harvey Sabinson, who began to edge his way "up" by teaching, first at Yale and now at CUNY, Queensborough, closer to his Long Island home. Two years ago Sabinson wrapped up 27 years of "flacking" (Variety's term) after 250 shows to become director of special projects for the League of New York Theaters and Producers. Between classrooms and committee rooms, Sabinson has completed a memoir, "Darling, You Were Wonderful." Sabinson explains his trade:

"Chances are it's the press agent who got you to the theater in the first place. Don't tell me it's the star or the playwright or the subject matter of the play. He or she (as many are women in this democratic endeavor) told you that such-and-such a playwright about such-and-such a subject. He or she did it by getting stories into the papers, by staging clever stunts, by composing an occasionally terrific, seductive ad, by arranging interviews on TV and radio, by having flyers printed, by trying to capture your interest in any way possible."

It is an amused book, because Sabinson never failed to be amused by his profession. For all the aspects of trickery and half-truths, theatrical press agents can be honorable men, and Sabinson is. When pushing a show, he'd he forthright: "This one is very light," he's say, "don't expect much." Or, "They're having trouble with this one and it'll be helpful to have your honest review." His approach was that of the amused guide.

He tells about his hits and misses through personalities. His trenchant anecdotes pin such as Barbra Streisand, Robert Redford, Lauren Bacall and Marissa Mell. Marissa Mell? She was "Mata Hari," one of Sabinson's and the National's most doomed attractions.

Which brings us to its producer, David Merrick. One cannot say - nor does Sabinson try to - that he put Merrick on the map. Merrick upt Merrick on the map. But it was through Sabinson for more than 20 years.

Their escapades included the time Merrick hauled out the New York phone books and tracked down noncritics with names identical to working critics. Merrick and Sabinson gave them free seats and supper at the Plaza after previews of "Subways Are for Sleeping," soliciting only comments about what both knew to be a musical the authentic critics would crucify.

From these fortuitously named guinea pigs, Sabinson and Merrick created one of those whopping "quote" ads intended to call attention to "Subways," but also to the absurdity of the ads themselves.

Slipping it into the then seven New York papers close to deadline, producer and agent were aware that careful copy readers could catch onto the trick. The Times did, but the Herald-Tribune ran the ad in its first edition. That was enough to have it reproduced as "news" around the world and do the ad's basic job, spotlight "Subways," which, Sabinson recalls, "survived until June when dismal word of month finally caught up with it."

The Better Business Bureau sternly denounced the trick as "deceptive and confusing," but Merrick was revealed to the wordly as a resourceful prankster.

Later Merrick startled Sabinson with another scheme: "I've got an idea for the greatest publicity stunt in the history of the theater, guaranteed to turn up long times at the box office."

Sabinson shuddered, Merrick resumed: "A great show that wins unanimous raves." Sabinson's secretary, Edie Smith, laughed uproariously over the episode. She first took dictation from a man who would have appreciated this example of booboseries, their historian, H.L. Mencken.

In "Darling, You Were Wonderful," Sabinson sketches the scope of an agent's responsbilities, including the ignored finesse of billing. Contracts may be signed guaranteeing two performers first billing over a title. Obviously, that's impossible, Sabinson explains how placement of type can satisfy three stars who have been guaranteed top billing. During the Philadelphia tryout of "I Can Get It for You Wholesale," the featured performer playing Miss Marmelstein asked: "Should I get a nose job?" "No," said Sabinson to Streisand.

My respect for Sabinson began early in his association with Merrick, then into his first major production, "Fanny," with Joshua Logan as his partner. After its long New York run, it was in tawdry shape by the time it got to the National and I so wrote.

The next week there was a Baltimore tryout I wanted to see by two Englishmen, John Osborne and Anthony Creighton, "Epitaph for George Dillon." It, too, was a Merrick offering and he told Sabinson: "Don't let Coe near it." Sabinson didn't tell me about Merrick's interdiction and produced seats for me himself. As it happened, I intensely admired "Dillon" and the New York critics didn't. It quickly folded but was revived very successfully there and elsewhere several years later.

That "Dillon" enthusiam, I later learned, lured some Washingtonians to Baltimore and decided Merrick that Washington was a good tryout audience. For a time his productions covered some six months of a National season.

Sabinson doesn't tell about that in "Darling You Were Wonderful," which suggests that he has many more illuminating yarns for some future book. I hope he's writing it. The only thing wrong with "Darling" is that it's not long enough.