TIME FLIES By Bill Cosby Introduction by Alvin Poussaint, MD Dolphin Doubleday. 176 pp. $15.95

One of the most sensational discoveries of the past decade, ranking right up there with the unmasking of adolescence and the true reason for monogamy, was the realization that people actually grow older as the years go by. The fact that the Earth revolves around the sun was no longer solely of interest to astronomers and science fiction buffs; if the '50s were the Age of Halitosis and the '60s were the Age of Noise, the '80s have become the Age of Age. I know this to be true because Bill Cosby, the bellwether, ex-counterspy and well-known father, has turned 50. He can no longer rely on himself to find his own car in a parking lot, and he claims that his body, once the temple of his spirit, has become a storefront church. But all is not lost. Following the trail so lucratively blazed by his previous effort, "Fatherhood," Cosby has written a sort of book about the experience, "Time Flies."

I use the phrase "a sort of book" advisedly. Mind you, I yield to no one in my admiration for Bill Cosby, an intelligent actor and a very funny man, and "Time Flies" is occasionally wise and often witty in a way that makes one laugh aloud on public transportation -- an exercise that is not recommended these days, except in London. It is also, alas, hardly a book at all, and not merely because it can be read in about an hour.

The Cosby style of humor depends on its fluency and ingenious self-mockery, but it also depends on a fairly thorough familiarity with the public persona of Bill Cosby himself. As with a single panel of "Doonesbury," a single Cosby routine is either meaningless or perplexing without its larger context, and a man from Mars, or for that matter a Frenchman, would be perfectly justified in wondering what all the fuss was about.

At his worst -- as in, say, a seemingly interminable essay on trifocal glasses -- Cosby resembles a thinking person's Erma Bombeck; at his best, when discussing the foods and physical surprises of middle age, he offers a keen insight into what might have happened if E.B. White had become a stand-up comic, which is not quite the compliment it may seem to be. No one who has read one of White's luminous essays could possibly want him to write anything else, which does much to explain the yawning abyss that separates comedy (White) from humor (Bill Cosby). Like tragedy, comedy is a body blow while humor is a shtick, a slap and (to mix metaphor with simile) too much shtick is like too much beer: After a while the audience stops getting drunk and starts getting full, which does much to explain the brevity of the Cosby oeuvre .

Still, there happens to be a genuine talent in there, or a credible imitation of one. The world is full of folk who have parlayed a small gift into a larger career, but the case of Bill Cosby is more complex. It is the case of a humorist who occasionally appears to be on the verge of perpetrating literature. The time may yet come when he does. After all, he's only 50.

The reviewer, a novelist and a contributing editor of Harper's magazine, is the author of "Bad Money," a study of business failures.