JEALOUSY. By Nancy Friday. Perigord/William Morrow. 538 pp. $19.95.

NANCY FRIDAY'S Jealousy is a big book in every way. Its length might seem daunting, but nearly every page is readable, intelligent and full of insight and information. Most of all, Jealousy is big in importance. Relatively little has been written on the subject, certainly for the general reader, and Friday is convincing in her argument of jealousy's central role in our lives and the ways in which our lack of understanding, even our denial, of the "green-eyed monster" often cripples our most intimate relationships.

Part of what distinguished Friday's earlier success, My Mother/My Self, was her personal involvement in her subject. She was interested in understanding her own relationship with her mother. The same holds true in Jealousy. Outwardly so successful that she would seem to have nothing to be jealous of, she was, she says, "Jealousy personified," often feeling deprived and abandoned, controlled and humiliated. "All my writing has been an effort to sort out the paradoxes of my life," Friday writes. Such personal investment in one's subject might lead a writer to focus narrowly on herself, especially in an area so full of defense and denial as this one seems to be. But Friday is so curious and so relentlessly honest that her quest for self-understanding is simply the starting point, the motivation that makes her want to know as much as she possibly can about the subject and to share it with us. The result is a quality of engagement and excitement that makes one understand why Friday's husband says he will have these words chiseled on her tombstone: "What does it all mean?"

While Friday offers her personal experiences at appropriate points, she concentrates on her research, incorporating the available theoretical work and interviews with a number of professional experts and with others, both friends and strangers, including the poet and novelist Erica Jong and the actress Zoe Caldwell, famed for her role as the jealous Medea, who tell their own stories of jealousy and envy. It is, in fact, a kind of chronicle of the process of understanding, as what Friday reads and whom she talks to lead her to new insights. Not without a certain amount of personal suffering: sometimes her work threatens her own unconscious defenses to the extent that she spends weeks bedridden with crippling back pain.

Crucial to this process is Friday's relationship with a New York psychoanalyst Richard Robertiello. In the several years that Friday has been consulting Robertiello's expert opinion, the two have developed a particularly intense friendship that, in its professional and personal dimensions, has such a high level of trust that they can be blatantly, even cruelly, honest with each other and still survive. "Provocateurs, battlers against one another's defenses," is how she describes them. "In our innumerable harangues over the years," Friday writes, "he has revealed himself to me as fully as I to him." When Robertiello, the one who has taught her that "Know thyself!" is "the necessary prelude to any life free of pain," insists that he is never jealous, Friday is perplexed and dismayed. But she is, characteristically, undaunted, and her attempt to probe Robertiello's psychological defenses -- and hence her own and everyone's -- becomes a narrative thread which moves the book along like a psychological detective story. To his credit, Robertiello, maddening and charming by turns, is up to the challenge, and each conversation between them becomes more and more revealing.

So what does it all mean? Friday makes an important distinction between jealousy, which arises from a fear of losing something we have, and envy, which is a desire to have something someone else has. We mistakenly use them interchangeably, but envy is by far the more destructive emotion and it is envy that is most often at work when we spoil what we love. When we lash out at the lover we think has betrayed us, we may not be so much jealous as envious of the lover's power over us.

FOR THE SOURCE of these envious feelings, Friday draws on the work of the psychoanalyst Melanie Klein, who broke with Freud in her emphasis on the importance of the infant-mother relationship in later development. In Envy and Gratitude, Klein theorized that adult envy has its basis in the infant's envy of the power of the mother's breast and the rage that grows from the desire to have that power for himself. It is a complex idea but one that Friday convincingly explains. Such rage, Klein suggests, is terrifying to the child who also sees the mother as the source of all gratification. No wonder we repress and deny feelinnvy in later life and yet are often compelled to recreate the drama. With "good enough" mothering -- the kind of mother who makes you feel as an infant that "you have the best seat -- the only seat -- at the Adoration Banquet" -- those destructive impulses can be converted to gratitude and love. As a friend of mine once deadpanned. "It's never not Mom." But when Friday talks about "mothering," she is really talking about a kind of care that can, and should, be given by fathers as well, and she has some interesting things to say on the subject.

Jealousy is about power, and Friday gives a good deal of space to the different ways in which, in our society, the struggle for power is felt and enacted by men and women. Recent changes in the roles of men and women have, she asserts, broken down our traditional defenses against jealousy, and unless we understand that, we are in for more trouble than we know. One thing that would help, she says: "The unhappiness described in this book would best be ameliorated if a child were raised from the day he was born with two sources of love, two sources of discipline, two models to imitate, and . . . two outlets to divide his inevitable rage."

There are occasionally points with which one might quibble, but trying to write a review of a book as full and complex as Jealousy reminds me of what a child I know once wrote at the end of a school report on American Indians: "A lot more happened but it's too much to say." The only solution, of course, is to read the book. It will, I think, change how we think about the way we live today and the way we might live tomorrow. CAPTION: Picture, Nancy Friday

JEALOUSY By Nancy Friday Perigord/William Morrow. 538 pp. $19.95 By Susan Wood

NANCY FRIDAY'S Jealousy is a big book in every way. Its lengt