Linda Winer Newsday theater critic and arts columnist Linda Winer.

Winer is chief theater critic and arts columnist for Newsday, which she joined in 1987.

There has always seemed something private, even unknowable about Barbara Cook. In the ’50s, this spectacular artist defined smart ingénue and vocal dazzler in “Candide,” “She Loves Me” and — her Tony winner — “The Music Man.” Now 88, she is the ultimate cabaret star, the epitome of exquisite, probing expressiveness.

This is not a guts-on-the-sleeve performer. Each song, especially ones by her favorite, Stephen Sondheim, is a self-contained piece of theater, flush with profound suggestions of life’s ups and downs, bright and deep, but never even close to look-at-me melodrama.

So what a surprise — actually, a thrill — to read her memoir, titled with a characteristic lack of fuss, “Then & Now.” Like her artistry, the book (co-authored with Tom Santopietro) is deceptively straightforward, clear-eyed, lively and frank. Even when she gets to the wrenching parts of her drama-packed story, the facts and feelings are laid out without a flinch.

“I’m so proud of myself for finishing it,” she told me in a recent phone interview. “I’ve never been a writer, never kept a journal; I express myself through song. Writing this was one of the most difficult things I have done, but now I actually have a book I can hold.”

And we have one to help us appreciate the back stories that doubtless help infuse her work with such specificity and power. In the preface, she speculates that, despite the many “amazingly beautiful things” that happened in her life, it’s “the bone-crunching crappy things that really shaped who I am.”

Take her opening lines. “I killed my sister when I was three years old. I was responsible for my father leaving us when I was six.”

She did not (her sister died of pneumonia, but Barbara may have passed on her whooping cough) and was not, of course, but this is how her needy, damaged mother made her feel about those catastrophes as she grew up, far from Broadway, in the Jim Crow South of Atlanta. And yet, she has “no memory of a time when I didn’t sing . . . I was born; I breathed; I sang.”

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She took clerical jobs, worshipped movies and, like every star-struck protagonist in hoary showbiz stories, got herself to New York. The rising action of her career seems perfect. Vernon Duke, the composer who wrote “Autumn in New York,” taught her how to eat her first artichoke. She hung out in a private midtown club where Judy Garland would stop in and sing for friends. From Garland, Cook learned that each song must have a beginning, middle and end, taking the listener on “an emotional journey” with “an unbroken line.” From Mabel Mercer, Cook says she learned to communicate the richness of good lyrics and ”really lay into consonants instead of vowels.” She says Mercer “didn’t have much of a voice but used words better than any other singer I can think of.”

Cook auditioned for the excruciatingly high role of Cunégonde in “Candide” with the entrance music to “Madame Butterfly” and, she remembers, “I sang the bejeezus out of that high D-flat.” She was in 17 Broadway shows, and won a Tony for her Marian the librarian in “Music Man.” She married and had a son.

And then she disappeared.

For most of the time between 1959 and 1968, in what she says should have been “my prime,” the pretty toast of Broadway was divorcing, drinking, overeating and severely depressed. “I was also unemployed and a drunk,” she writes, “not a nice, ladylike drinker, but a drunk. I just stayed home and got drunk every night by myself . . . I didn’t shower or brush my teeth for days at a time.”

When she reappeared, she was no longer the petite young star.

She almost didn’t sign the contract for her Carnegie Hall solo in 1975 because she says she was embarrassed about her weight and terrified about doing a solo show on that august stage. But the concert, her first with her beloved arranger/accompanist, Wally Harper (who died in 2004), was a smash. What she remembers about The New York Times rave, besides the praise, was that the paper ran contrasting photos of her — “one when I was slender, and one after all the weight gain. I would go on television talk shows and the hosts didn’t want to talk about music — they wanted to talk about weight.”

The producer of Johnny Carson’s “Tonight Show” refused to book her because, as she puts it, “he didn’t want fat white women on his show.”

She has been sober since 1977 and her cabaret career has been soaring ever since. Michael Feinstein, cabaret legend and recent co-owner of Feinstein’s/54 Below, the club next to Studio 54, said in an email, “Working with Barbara has been a highlight in every way and I simply adore her. She is not only a singer’s singer and supreme interpreter of text, she is also sweet, passionate, bawdy, funny and wonderful company.”

“What’s so good about cabaret,” she told me, “is that you run the show. You sing what you want to sing. You sing it the way you want to sing it. You have control.”

She only returned to a Broadway show once, as part of the 2010 “Sondheim on Sondheim” — a kind of docu-musical on his 80th birthday. “Lately I’ve been comparing him to Shakespeare,” she confides to me. “I don’t know what he thinks about that, but the depth of his work is so great that there’s something new every time.”

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During the run, her physical problems made it hard for her to dash up and down the backstage stairs. “So I sat in the wings and got to listen to his work. What a pleasure.”

In 2011, she became a Kennedy Center honoree. Last spring, she canceled a one-woman Off-Broadway show, also called “Then & Now,” at the last minute because the pressures of finishing the book were reportedly too much. She will be at Feinstein’s/54 Below, the cabaret next to Studio 54, July 21 and 23. The shows have long sold out, but the website says you can call to be put on a waitlist. “After that,” she says, “I’ll plan other things.”

She said she doesn’t see much Broadway these days because it’s hard for her to get around. She has had trouble walking for about a year and uses a wheelchair. But she says she had intended to see the revival of “She Loves Me” (it closed on July 10), which she heard was very good. I asked if she used to check out revivals of her shows. “Through the years, of course,” she answered with what sounded like a wicked smile. “I always think they should have done it my way.”