Even now, Peter, Paul and Mary can’t be stopped.

During the early ’60s folk boom, their entwined harmonies seemed to be a part of every progressive rally, and since then, they have gained new generations of fans through numerous tours and PBS specials.

Although Mary Travers died of leukemia, at 73, five years ago, the trio’s legacy goes on, in a new CD, in another PBS special and in a new coffee-table book, “Peter, Paul and Mary: Fifty Years in Music and Life” (Charlesbridge, $29.95), which covers their long career largely through photos.

And photographs tell the story well – especially in performance, with Mary’s striking blond hair and ethereal look set off by the two studious-looking men.

“She was the spotlight, and she was luminous,” Peter Yarrow, 76, says over the phone from New York. Or as it is put in the book: “Mary’s stage presence and her dramatic beauty were as arresting as her remarkably expressive and powerful voice.”

Longtime fans may notice that the title of the book, out Nov. 4, is off by at least two years, given that the trio came together in 1961 and released its first album in 1962. “It’s a celebration of 50 years together, and it took a couple of years to get it together,” Yarrow says.

They titled their 1968 album “Late Again,” “because we notoriously ran late,” Noel “Paul” Stookey, 76, says from his home in coastal Maine. “In fact, the back cover photo has us standing in front of a chalkboard with the three having written 100 times: ‘I will not be late again.’ ”

Even so, here they were, two or three years after the 50th anniversary.

“Part of it is, you can be a pretty good judge at how long you’re going to take at something, but the moment you move into collaboration, and you honor the other people in the collaboration, then all bets are off,” Stookey says.

Another unusual touch is that Peter, Paul and Mary are all credited as the book’s authors.

Yarrow and Stookey’s original intent was to write a book of individual reminiscences, but that idea was abandoned early on. “Somehow the absence of Mary struck both of us as odd,” Stookey says, “because in many ways she had spoken at many of the events that occurred; she had already articulated her feelings about some of the major experiences that we had shared. So, ultimately, the most truthful way of presenting this story was collaborative, with no single voice being codified.”

The process was one of consensus, and Yarrow and Stookey incorporated some of Travers’s writing. (She has a posthumous book out this year, “Mary Travers – A Woman’s Words.”) Stookey says “Fifty Years” also includes “a literal rendering of her onstage monologues.”

The book’s many photos include images from early rehearsals and performances, vividly conveying one quirky aspect of their stagecraft: They always ran on stage, hand in hand.

“I guess it started with a sense of joyfulness, rather than . . . dignity,” Yarrow says with a laugh. “It was natural. It was never a discussion, never something that we were told to do. Except at the end, when Mary was in wheelchair and with an oxygen tube. We didn’t run with the wheelchair. But she insisted on performing in those last months, when she was wheelchair-bound. She wanted to do it.”

The trio was a huge hit at the dawn of the ’60s, with such songs as “Lemon Tree” and “If I Had a Hammer” charting as singles. Their first album, in 1962, was in the Top 10 for 10 months and in the Top 20 for two years. At one point in 1963, all three of their albums were in the Top 10.

“We never went into a studio thinking we were going to record hits,” Stookey says. “We found songs, or songs found us, that were very moving.”

As would be expected in a show-business picture book, there are shots of the three with other famous people: performing with Andy Williams, chatting with Bob Dylan or Richie Havens, meeting the Beatles and Ed Sullivan while in London to perform for the queen.

But made more conspicuous in the book is their place at so many historic moments of that half-century, from the JFK inaugural to the 1963 March on Washington. If there was a progressive struggle or rally, they were there: Vietnam War protests, the civil rights march from Selma to Montgomery, Ala., Kent State, the strawberry fields of California, Earth Day celebrations, and demonstrations against nukes, U.S. involvement in El Salvador and apartheid in South Africa.

“We were there to participate in something that was bigger than us, and that allowed us to pledge ourselves wholly to the event and to the moment,” Stookey says.

After all, they weren’t merely entertainment, Yarrow says. “You are there to be as much a participant as any of the speakers. You’re there to be as much a participant as any of those marchers,” he says.

Looking at the book’s photos of those events, “you begin to get an inkling of the connectiveness,” Yarrow says. “Somehow there was a thread in music that made people think they were part of a real positive shift in the world and in their country and their participation would make a difference.

“This was a path that was forged by Pete Seeger, who made every appearance that he made resonate with the conjunction of music and his dreams and hopes for a world that was just and fair and humane and equitable and, in the last years, that would survive.”

In the book’s foreword, Secretary of State John F. Kerry says of the trio: “Their songs became the soundtrack of my life and of a generation. They changed the cultural fabric of this nation forever.”

“Fifty Years” is arriving just ahead of a PBS pledge special in December and the release of a new archival recording of songs that the trio sang in concert and that hadn’t made it onto earlier albums.

“I keep telling my wife, you’re going to be hearing a lot of Peter, Paul and Mary in the next 21 / 2 months,” Stookey says.

Even so, these projects do not mark the end of the line for the trio.

“The work goes on, “ Stookey says. “Mary’s phrase, ‘To be continued . . .’ concludes the trio’s part of book.” And the two surviving members remain active on a number of issues, including climate change and Middle East peace, and they continue to sing, individually and together.

The two sang at the University of Maryland at College Park in late September at a concert honoring longtime folk music personality Dick Cerri. Late last year, they sang at the Washington National Cathedral memorial for Nelson Mandela and, the night before, at a fast on the Mall by advocates of immigration reform.

“Noel and I sang together with the parents of Trayvon Martin and the father of one of the children that had been killed in Newtown [Conn.], and the 50th anniversary of the March on Washington,” Yarrow says. “So it’s not just the spirit of the work continuing but literally the work itself. “

And wherever they are, Travers is there, too.

“When you hear us sing together,” Yarrow says, “you will hear the spirit of Mary in our voices. It’s there! We sang together for almost 50 years. So we’re carrying it on not just in terms of the intention, but we’re carrying it on, quite literally, with that energy and that history and that lingo, that shared syntax, that way of thinking.”

Catlin is a freelance writer.