At this point in music history — more than a half-century after popular music was re-defined by the youthful energy of rock ‘n’ roll — one important measure of greatness is longevity. Many of the idiom’s greatest artists have achieved it: Bob Dylan, Leonard Cohen, Joan Baez, Paul McCartney, Joni Mitchell — and Judy Collins.

Those above-named artists now range in age from 67 (Mitchell) to 74 (Cohen).

Collins is 71, but if anything, her “elder” status only seems to have inspired her to become more prolific. Last year, she released a new album, “Paradise,” and a children’s book, “Over the Rainbow.” She still does about 110 shows a year, by her estimation, and she is now in the final editing stages of her upcoming autobiography, which she says will be titled “Suite: Judy Blue Eyes — Sex, Drugs, Rock ‘n’ Roll and the Music That Changed a Generation.” The book is slated to be published in September.

“I’ve actually been working on it for four years,” says the folk-pop icon, who co-headlines the second-night lineup of the Ann Arbor Folk Festival on Saturday at Hill Auditorium. “It starts in 1968, and spans the 20-year-period after that. The year 1968 really is the centerpiece, though. There’s a lot to be said, still, for the events of that year.”

One of the events of that year was her romance with Stephen Stills, which Stills immortalized in the song that inspired the book’s title.

Collins has already written several books over the years, including a couple of autobiographies, a novel and a couple of books about being a survivor of suicide: Her son took his own life in 1992. “I really wanted to write this particular book at this time, though, because I knew I would be turning 70,” says Collins by phone from her home in New York City.

Collins’ career as a recording artist actually dates back 50 years — her first album, “Maid of Constant Sorrow,” was released in 1961. Initially, she was an interpretative artist, who lent her sparkling, winsome soprano vocals to songs written by the folk-music poets of the ‘60s — Dylan, Phil Ochs, Tom Paxton, and, later, Mitchell and Cohen. In the late ‘60s, she began writing her own songs to complement her expressive covers of other writer’s tunes.

But she’s always continued to flex her interpreter’s chops. “Paradise” includes her own versions of classic and / or vintage tunes like “Over the Rainbow,” “Ghost Riders in the Sky,” Baez’s “Diamonds and Rust,” Paxton’s “Last Thing on My Mind,” Jimmy Webb’s “Gauguin” and Tim Buckley’s “Once I Was.” She also puts her own stamp on “Weight of the World,” by Amy Speace / Jon Vezner / Jud Caswell. (Speace and Vezner appeared at The Ark last month as part of the Decembersongs show.)

In fact, her recordings of “Diamonds and Rust” and “Last Thing on My Mind” are duets with a couple of old pals. She duets with Baez herself on “Diamonds.” And, fittingly, she and Stills do a duet on “Last Thing” — a song in which the singer laments a broken romance and expresses his / her regrets.

“Stephen and I remained friends, and we’d talk to each other on the phone every now and then, and then we had dinner one night in California in around 2007, and we began talking to each other more often after that, and he just suggested one night, ‘Why don’t we do a duet?’ And that just seemed like a very good song for ex-lovers to sing. My husband took photos of us singing it in the studio — it was like something out of a Robert Altman film,” she says with a laugh.

Their performance of this very poignant song is quite stirring, and both are in fine voice.

As for the Baez duet: “I started performing that song live in ’09, and one day I called Joan and said, ‘I want to record this for my new album,’ and she said, ‘Oh, that’s great, no one ever records my songs.’ Then she suggested we do it as a duet. We also performed it together at the Newport Folk Festival last year, without a rehearsal, in the rain,” says Collins, laughing again. “It’s on You Tube.”

Collins played a key role in helping to launch the careers of Mitchell and Cohen, in 1966-‘67, when she recorded their songs “Both Sides Now” and “Suzanne,” respectively, before either of them had a recording contract. At the time, Collins was already an established, successful artist, so her recordings of their songs exposed each artist to the mass audience — and eventually led to each landing a record deal.

So we couldn’t resist asking Collins about her recollections of how those recordings came to be, and her interaction at the time with Mitchell and Cohen — who, over the decades, have come to be recognized as two of the greatest songwriters of the last half-century.

“Well, with Joni, she was still playing the clubs at the time — she wasn’t on the marquee anywhere, but some people had taken notice,” recalls Collins. “And one night I got a call from Al Kooper, who was at Joni’s house, and he said, ‘Joni has this song, and I think you should hear this,’ and she sang it to me over the phone. Of course, I loved it.

“With Leonard, I also got a call, from a friend who said, ‘I know this guy, he’s Canadian, he’s a poet, and he wants to come play these songs for you.’ So Leonard came over, and he said, ‘I want to know if you think these are songs.’ And he played three songs for me — ‘Suzanne,’ ‘Dress Rehearsal,’ and ‘The Stranger Song.’ “I said, ‘Of course they’re songs, they’re wonderful songs, and I’m going to record them.

“I’ve always thoughts that part of my function is to expose people to other artists who they might not have heard of. So, to this day, I still like to keep my foot in the interpretive camp.”