IF PRESIDENT CARTER would only listen to Robert Kaiser and Jon Lowell instead of Pat Caddell, he could stop losing sleep over "the crisis of confidence" in America. Kaiser and Lowell-- reporters for The Washington Post and Newsweek respectively -- have gone out with little tape recorders and big questions to study the great American middle class, what they call the "two-car, boat-in-the-garage, cabin-at-the-lake class." They have returned with a kaleidoscope of interviews and the news that "a large portion of Americans think that their lives are pretty terrific. . . .they seem to be having one hell of a time."

Middle America they define as the 110 million wedged between the rich 5 percent up top and the 40 percent at the bottom. An annual income of $18,500 will put you in this strata which, they say, is now the largest slice of America and sets the tone "for an entire civilization" -- including our fast-food, shopping center culture based on the automobile and the television set.

Kaiser and Lowell have mostly tracked this species to what they consider its favorite haunt -- Las Vegas. Although they offer no "irrefutable scientific evidence," -- only "theories born of reporters' instincts" -- they pronounce the ordinary folk out there to be optimistic, contented, satisfied and solidly in favor of the status quo. So much for diminishing expectations and "the national malaise." Beyond that, Kaiser and Lowell refuse to speculate on what their findings mean -- politically or otherwise. "Where this may lead is a mystery to us," they say. "We have ducked a lot of hard questions." Instead of answers, they offer "10,000 different impressions, plus a dash of mischief and a lot of fun." Meaning, apparently, that this slide show is anything you want to make it -- so just enjoy.

Politics and politicians, the authors say, do not much interest middle-class Americans. What does are some quaint notions that many people think went out with Chester A. Arthur -- belief in the efficacy of hard work, for example. Battista Locatelli, whose zest for 20-hour working days has made him rich, told the authors, "By nine o'clock in the morning, I think the day has already went by." He started his Las Vegas restaurant with $17 and now, seven years later, has a million dollars, his private jet and a hair transplant -- and, I'd say, a ticket out of the middle class.

Old-fashioned ingenuity, ambition and optimism are still alive in Kaiser and Lowell's Middle America. And so is a yen for new and improved products -- all kinds. To hear them tell it, hair plugs and silicone implants are becoming as ordinary as snowmobiles and CB radios. George Tippit, an M.D. in the big breasts business, grosses half-a-million dollars a year in implanting silicone in two to three thousand Middle bosoms. He operates in the shopping center of the MGM Hotel -- where else? -- and describes his speciatly as 10 percent surgical technical and 90 per-cent "artistic sense of proportion."

Offhand, Las Vegas might not seem like the average, everyday, middle-class community. Kaiser and Lowell say they chose it for most of their interviews because it is "so perfectly American," because its trailer parks, convention halls and gambling casinos draw more visitors than any other resort in America and "an extraordinary percentage. . . seem to fall into our Middle American category." An extraordinary percentage also seem to be oddballs -- more than one might expect to find, say, in St. Cloud, Minnesota or Peoria, Illinois. Wilma, the Las Vegas hooker, for example, who thinks of herself as a therapist: "I make my liveing talking. . .conversation is the key. Afterwards I always tell them they were somebody special. . . .It's kind of like I'm not selling my body. I'm helping them to like themselves." Or the Reverend Sharp who preaches "a positive gospel" in an old Las Vegas Cinerama movie theater and plans to build a "spiritual shopping center" which will include a hotel, a Bible college, "a Christian nightclub" in a space needle with a wedding chapel at the top. But not, he assures the authors, "a fast buck operation."

Kaiser and Lowell do not take a dim view of all this. They take no view at all. Their book, they say, is "like a documentary film in words . . . . There is no moral lessons in this movie . . . . We are not going to pass judgments on McDonald's hamburgers." They are simply offering "food for thought: life stories and anecdotes," and offering them in a bemused, slightly gee-whiz manner. If they are troubled by the signs of narcissism and materialism that seem to be the hallmarks of their middle class, they aren't saying. What they do say in this good-natured, readable book is that Middle America has never had it so good. What that means for everybody else remains to be seen.