IT WASN'T very long ago that insomniacs were a lonely, over-looked bunch. No one paid much attention to you as you tossed and turned and fiddled with your radio dial, your depression increasing each time you heard the National Anthem at the close of another station's broadcast day. And then you just lay there and tried to sleep.

But eventually, the national media began to cater to the previously ignored all-night market with shows like "NBC Overnight" and Boston's "Five All Night Live." And the leader of the late-night media circuit for the last four years has been a show that keeps millions of Americans from coast to coast glued to their radios into the wee hours of the morning--Mutual Radio's "Larry King Show." Five nights a week, King hosts a bona-fide, nation-wide call-in talk-show that runs from midnight to 5:30 a.m. And what he has written in Larry King, his jumbled but entertaining autobiography, is the story of how young and restless Larry Zieger of Brooklyn became the star of what he calls a nightly "national town meeting."

King exposes it all here--in case you wanted to know it all. He tells of his three divorces from two women, of his ambition to be rich and famous--"to be a big man." He writes of his irresponsibility with money, and how he once asked his friend, Richard Nixon, to save one of his creditors from legal troubles. The deal subsequently led to King's arrest and put an end to his Miami broadcasting career. He tells what it's like to host the "Larry King Show," and he reminisces--at excessive length--about his adventuresome childhood in The City.

Much of King's account is meant to be publicity for his show, and he describes its format, guests, and its phenomenal rise in popularity. When it went on the air in 1978. "The Larry King Show" was broadcast on 28 stations. Today, more than 250 Mutual affiliate stations carry to show to five-and-a-half million of what King affectionately calls "King-a-holies." Calls to King have made the Virginia studio's area code the busiest in the nation during the late night shows. "In the beginning," King recalls, "there was more than one occasion when [area code] 703 blew--no one could make a call in an area that includes the Pentagon and the CIA."

Most callers try to get through to King's guests, who have included a broad range of the powerful, rich and famous in America. King interviewed President Gerald Ford in the midst of the confusion at the 1980 Republican national convention. He was the first interviewer in America to speak with a returned hostage from Iran. Senator Henry Jackson (D-Wash.) drove to the radio studio at three o'clock in the morning and blasted President Jimmy Carter the night the news broke of the aborted attempt to rescue the hostages.

Danny Kaye, Walter Mondale, Mel Brooks and several Cabinet members and senators have visited the show. And all of these guests are grilled by hundreds of callers from all over the country. "That's what the show offers," King writes. "A chance for people...to put their questions directly, as equals, to our guests."

But some callers don't just call "The Larry King Show" for its guests. Like any talk show host. King draws his share of weirdos. Usually he cuts them off. But he has a handful of regulars, such as "The Portland Laugher," a caller who never speaks, but only laughs into the phone. "I'll say, 'Sir, what do you think of President Reagan's economic plan?' and The Laugher practically passes out from laughing," King writes. One wonders whether the listeners do, too. Another regular labels himself "The Syracuse Chair," and claims to be the voice of the empty chair that was to stand on the stage in a 1980 Reagan-Anderson debate in which Carter refused to participate. "I'm thinking of giving it a try in '84," he once told King. "So I have to start looking for an ottoman to run with me."

King has interviewed just about everyone famous in America over his long career as a radio broadcaster in Miami and on his current show. In his book, he discusses friendships with Jackie Gleason, Lenny Bruce, Don Rickles, and Nixon. He recalls interviews with the likes of Bob Hope, Bill Cosby, and Milton Friedman.

King repeatedly reveals his own life-long obsessions with fame and wealth, with Cadillacs, beautiful women, and horse races. "Miami was a personality town," he says in the very first paragraph of the book, "and I was a personality." Though King claims to be over his obsession, the reader wonders whether this book about his life with the stars is only another manifestation of his drive.

The book is disjointed and incoherent. It lacks any real direction and becomes a series of anecdotes--albeit interesting ones--chronological accounts, testimonials, and personality profiles. He seems to have told all kinds of stories to his ghost writer, Emily Yoffe, who tried to piece them together in some orderly manner.

King often lapses into a conversational tone. After all, he is a conversationalist, a veteran talk-show host who gets paid--and paid well--to speak. The book is at its best when it becomes casual and King sounds as if he's speaking.

THE MAIN QUESTION about this scattered, disjointed collection of anecdotes and memories is: Why did 45-year-old Larry King, who talks about himself to millions of Americans five nights a week, write a premature autobiography at all? Publicity for the show is one obvious reason. But there must be more--and it is apparent in King's tone both throughout the book and on the air: Larry King is an egotistical semi-star who only lives in the shadow of the Frank Sinatras and Jackie Gleasons and Lenny Bruces with whom he has mingled throughout his career.

"As a kid," he says, "I couldn't have fantasized about the life I have today: I had no idea this sort of life existed. Sometimes, I have to admit. I can't believe it exists for me, that I'm living the American dream."

Five nights a week, Larry King speaks to and for America. He is a superb interviewer, a truly enjoyable radio personality. But his self-aggrandizing, self-consciously honest autobiography only makes one wonder about this man's idea of America and its dream.