A Song at Twilight : I’LL THINK ABOUT THAT TOMORROW <i> By Evelyn Keyes</i> ; <i> (E. P. Dutton: $19.95; 339 pp.) </i>

In her earlier memoir, “Scarlett O’Hara’s Younger Sister,” published in 1977, Evelyn Keyes led us a merry, sexy dance through her marriages and other romantic entanglements in Hollywood. It was all the merrier because the writing was so manifestly her own. No ghostly hand could have achieved such a perky, personal, uninhibited voice, recalling an uninhibited and unembarrassed life.

Having been under personal contract to Cecil B. De Mille (“The Buccaneer,” “Union Pacific”), she did indeed play Suellen, Scarlett’s kid sister, in “Gone With the Wind,” and was memorable in “Here Comes Mr. Jordan,” “The Jolson Story” and “Mrs. Mike,” among dozens of other less memorable films. She did a cameo in “Around the World in 80 Days” for Mike Todd, with whom she spent three years.

Now, 14 years after the first reminiscences and at the age of 71, Keyes has evoked GWTW again for the title of another memoir, “I’ll Think About That Tomorrow,” Scarlett’s defiant shucking off of all that is worrisome.

The Keyes voice remains unchanged: perky, candid, funny, self-chiding, alive with spontaneity and joie de vivre. But there is a central difference. Beneath the amusing tone, her book is a thoughtful portrait of an actress who is no longer an ingenue coming to terms with her own seniority.

She insists that someone really did ask, “Didn’t you used to be . . . “ And there was the child in London whose mother reminded him that they had watched one of Keyes’ films on the telly only the night before. “You look older now,” said the honest child. The mother was horrified, but Keyes says she praised the boy for his candor and his accuracy.

Not that there haven’t been the odd flirtations and flings, a nice noncommittal time with an urbane Englishman in London, a curiously short-circuited acquaintance with an art dealer evidently uncertain of his preferences.

“Me! Of the pausing menses,” she remarks of her life at one point.

She gives an often hilarious account of her ongoing friendship with her former husband Artie Shaw, and the memoir begins as they sell the house they had so carefully built atop a sea cliff in Spain, to resettle in a vast, 25-room mansion in Connecticut. The portrait of Shaw--demanding, irascible, an obsessive enthusiast about what grabs him at a given moment--is extremely affectionate, but it does suggest that one of Keyes’ gifts is for infinite patience. (She gave a warm and touching account of the years with Artie in Brigitte Berman’s Oscar-winning documentary about Shaw, “Time Is All You’ve Got.”)

There is a sad, affectionate account as well of her later, post-marriage meetings with John Huston, whose wasting battles with emphysema convinced her to give up cigarettes cold-turkey. Huston, always gallant, was disappointingly noncommittal about the writing in which she had begun to take such pride. (What emerges from the book is the observation that Keyes always has been drawn to men with notably strong egos. It makes her own declaration of independence, which the book represents, the more impressive.)

For in the end, “I’ll Think About That Tomorrow” is not so much an anecdotal look back as it is a story of learning to cope with solitariness and with an acting life that in a sense begins again when, if you can, you start playing the star’s aunt, mother or, depending on the age of the star, grandmother.

Keyes has coped very well. She toured successfully in a revival of “No, No, Nanette.” Having begun in a chorus line, she took up dancing again, getting up to speed and keeping in shape by rehearsing tap routines on William and Talli Wyler’s tennis court (which Fred Astaire had built).

Primarily, she has coped and prospered through her other formidable talent, writing. She published a novel, “I Am a Billboard,” in 1971. Besides the two volumes of autobiography, she has written a screenplay that might well get made one of these days. She has spent several years working on a historical novel, happily burying herself in research at libraries at UCLA and elsewhere. The novel has been shelved temporarily but probably not permanently.

What gives her book a relevance beyond the star-memoir genre is that the fears and difficulties of adjusting to a changed life are universal. Her early dreams--Hollywood and finding a prince, or several--had been realized. Now, she writes, “The only trouble was I hadn’t prepared for anything, didn’t have a single dream whatsoever to carry me through my later span.” Many a woman could say that, who hadn’t played Scarlett’s little sister.