FOR ELLEN DOUGLAS this novel, her fifth, is a startling and entirely impressive departure. Her previous work is, for all its indisputable excellence, faithful to the themes and conventions of the "Southern novel"; A Lifetime Burning, though it happens to have an unspecified Southern locale, could be set anywhere. Her previous work follows, on the whole, traditional narrative forms; A Lifetime Burning is cast in the structure of a diary and shifts back and forth between dream and "reality." Her previous work deals primarily with social questions, especially those touching on relations among blacks and whites; A Lifetime Burning is an "interior" novel that explores intensely private matters and that uses male and female homosexuality as a metaphor for the emotional distance between the sexes.

Its diarist, Corinne, is 62 years old. She has been married for three decades to George, a physician, and has given birth to five children: two sons and a daughter, all now grown and gone, and twins who were stillborn. In the oppressive heat of August she begins the diary, a confessional document that she hopes will "tell the truth" to her children: "I want to say everything at once, to understand, to accept, to stop striving. Also, I want to say--nothing. Or, to put it another way, I want to explain everything truthfully and at the same time to be always right, always charming, always lovable, always beautiful. Is that too much to ask? Especially when I'm doing the explaining."

What has set her on this chain of recollection and reflection is her husband's sudden, inexplicable decision to stop sleeping with her. He has always kept her at an emotional distance, but this physical distance baffles and demoralizes her: "It wasn't at all that he was impotent. He was impotent with me. He didn't just not feel even the mildest interest in me, he was repelled by, he loathed me." She discovers, she tells the diary, that he has been having an affair with a neighbor:

"Here's the most outrageous, the most humiliating thing about it. She's unattractive. A toad. Never mind that she's fifteen years younger than I am. She is so short she can walk under the dining-room table without bending her head. Slight exaggeration. Really, she's four-eleven. But she has the look of someone who barely missed being a midget -- arms a trifle short, a short- legged waddle for a walk. And that's the least of it. What's far worse is the smarmy, horrifying, idiotic, doting smile, the way she opens her eyes wide (she does have expressive eyes), lips parted (good mouth, too), and gazes up at him as if he's the cleverest, most wonderful thing that ever came down the pike."

But this description, vivid and detailed though it may be, is a mere ruse; George's female lover, "the Toad," is a dream. Reality, which Corinne at last forces herself to confront, is worse: George's lover is a man, a lab technician at the hospital where he practices, a person of limited education and no apparent charm or appeal: "What is it? I can't put my finger on it. That -- That he chooses at last, for the one to whom, after a lifetime closed off from those who love him, to open his life to, someone uneducated, limited, desperately, fatally flawed and wounded, ill, feeble -- small."

This may be reality, but it is not all. Slowly, fearfully, Corinne edges ever more deeply into her confession. She describes her long, passionate affair, in the years before her marriage, with a man who was himself married. She recalls George's courtship of her, her acceptance of him as a partner of convenience -- and her unexpected falling in love with him six months after their wedding. But all of this is prologue to her disclosure in the diary's final entries of her affair, two decades earlier, with a married woman -- an affair tha ht leads her to what she believes to be an understanding of "the terrible, irremediable reservations between men and women that make us feel sometimes that we must be different species." When the affair ended, she writes, she was devastated: "Everyone knows what it's like to lose a mate, a beloved friend. I felt as if someone had thrown a rock at me, struck me in the temple, when I was riding an escalator. I had to keep moving with the moving stairs. I staggered, scrambled to keep myself upright, hung onto the side rail. Some days I didn't care if I got to the top and went through the crack and never came out."

Or so she says. Because what we do not know for certain, even in the final pages, is whether this "reality" is in fact yet another invention, yet another distortion of "truth." In one of the final entries in her diary Corinne writes: "Did you know that some psychological study or other proves statistically that the average human being tells between twenty-three and one hundred and thirty-seven lies a day?" Lies, distortions, deceptions, evasions -- these are essential to the maintenance of the delicate fabric of which family and society are made.

This, as I interpret it, is the central theme of A Lifetime Burning: we are separate beings and cannot be otherwise, we are mysteries to each other and will always be. In order to keep the structure of our lives intact, it is necessary to withhold the full truth; we invent ourselves for others -- and, perhaps, for ourselves. This is what Corinne wishes she had been told when she was young: "Ah, but you see how it is. We're not what we seem. It can't be helped." E.M. Forster's passionate plea -- "Only connect!" -- can never be fulfilled.

To say this is not also to say that A Lifetime Burning is a gloomy or despairing novel; to the contrary, Douglas finds a realistic measure of hope in the conclusion that our illusions are what sustain us. And the novel is a splendid piece of writing, fusing dream and experience into a seamless narrative that acquires a remarkable urgency as the "evidence" accumulates. At a stage in her life and career when most American novelists are content to repeat themselves, Ellen Douglas has struck boldly into new territory and has come back from it with her finest novel.