As time passed, I learned the Rule of the Great: When someone you greatly admire and respect appears to be thinking deep thoughts, he is probably thinking about lunch. --Joseph D. Reed Jr., ambassador-designate to Morocco

Lunch -- to the naive -- is something you eat.

But to New York writer Louise Bernikow, lunch is life in microcosm: a midday me'lange of power, passion, politics, full of sound and fury, signifying everything.

"Lunch is not a meal," she sniffs, sipping Perrier and scanning the (lunch) menu at a downtown restaurant. "It's a game. Played with expertise and gusto, it can become not just the high point of a business day, but sometimes even better than life itself."

Bernikow, 40, has so thoroughly studied American noon-hour habits that she just might become a legend in her own lunchtime. Her research included countless lunch hours with a wide variety of lunchers, from the Double-Breasted Executive to the Metropolitan Gossip.

She interviewed non-lunchers who munch a sandwich at their desk ("They spend most of their time fantasizing about people who have gone out to lunch"), along with some of the nation's top lunchers ("The ones who treat lunch with the solemnity of High Mass").

The result was not weight gain ("I jog"), but publication of Let's Have Lunch: Games of Sex and Power (Crown Publishers, 115 pgs., $7.95). The book covers everything from "The Invitation" to "Arrival and Positioning" to "The Moment of Truth: The Expense Account."

"Eating is merely consuming food," she concludes, "but lunch is an art form."

A former Fulbright scholar who classifies her previous books (The World Split Open and Among Women) as "scholarly," Bernikow admits she wrote Let's Have Lunch, "because I was hungry.

"I was having what's known as a 'fishing lunch' with my publisher. That's where writers and editors fish for ideas. I was trying to sell him on a historical epic I wanted to write, and he didn't exactly love the idea.

"But people never say 'no' at lunch, because there's more of a congenial atmosphere than in an office. Which is one reason why people go to lunch. So instead of turning my idea down, he didn't say anything."

During the long silence, Bernikow looked around the dining room and started to laugh. "I saw two people having the exact same lunch we were, and I pointed it out to my publisher. He said someone should teach a course on lunching in business school, and I said I'd like to write the manual.

"He looked at me, I looked at him, and we knew we had a book."

She also had a perfect example of Bernikow's "Law of Fishing Lunches: You Never Land What You Hoped For, But If You're Lucky You'll Hook Something."

A self-described "late-blooming luncher," Bernikow proclaims: "Lunch is a political act, a spectator sport, a ceremony, a recess, a delight. People who lunch plan it in advance, worry about it, feel like wallflowers if they're not doing it and live from lunch to lunch."

As proof of America's love affair with the ritual, she points to "the riot" following Jimmy Carter's proposal to eliminate the Three Martini Lunch. "That caused a far greater outcry," she claims, "than when Reagan proposed cuts in Social Security and school lunches. Business people need that lunch."

Some of the business conducted at lunch, she notes, is a racier variety. "According to a survey done by Cosmopolitan magazine," she says, "the highest incidence of extramarital sex occurs between noon and 2 p.m."

The possibility of lunch becoming erotic, according to Bernikow's Naked Lunch Law, "is directly proportional to the attention paid to your underwear in the morning."

For erotic lunches -- and any other kind -- Bernikow considers Washington one of the top Big Lunch cities in America. "All the decisions here are made over lunch."

The difference between a Washington and a New York lunch?

"In Washington, lunch timing is a bit more rigid. People actually have to get back to committee meetings and hearings. In New York, lunch is more open-ended."

The biggest mistake of the neophyte luncher, says Bernikow, is "bringing up business when you sit down. Then, if things don't work out, you're left with lunch on your plate and the prospect of a very uncomfortable hour."

The proper time "for the purpose of lunch to become manifest," she says, "is when the main course is consumed, but before dessert or coffee are served." The French "who know a great deal about the rhythm of lunch," call this moment "Entre le poire et le fromage."

In a meal lacking both pear and cheese: "The moment occurs anyway."

The latest in lunch trends, says Bernikow, is "ordering macaroni and cheese . . . Reagan's favorite. I hear he tops it with jelly beans. You don't see brown liquids at lunch any more -- no bourbon, dark beer, rye. Everyone's drinking white wine, spritzers and mineral water."

The juiciest lunchtime gossip: "David Stockman is a 10 percent tipper. I heard it at the baggage claim of National Airport from a man whose son is a busboy at a restaurant where Stockman eats."