The Summer of Love didn’t just happen in a vacuum nearly 60 years ago. It wasn’t a kind of spontaneous combustion that created the hippie movement and the peace and love ethos of the ‘60s generation.

“The Last Great Dream: How Bohemians Became Hippies and Created the Sixties,” a new book by historian Dennis McNally, explores the roots of the counterculture that flowered in San Francisco’s Haight-Ashbury in 1967, giving birth to what the author describes as “the world’s first psychedelic neighborhood.”

“What I tried to do is give a broad overview of where hippies came from,” McNally told me when I interviewed him last week. “A broad but very detailed story of where these things came from is the only way you can look at cultural history.”

McNally and I will be in conversation about the book at 6 p.m. May 28 at the Mill Valley Community Center. The evening includes music by the Flying Salvias and Beat poetry by J. Macon King, editor of the Mill Valley Literary Review. Admission is $20 and $55.20 with the book. Get tickets on eventbrite.com.

The 76-year-old, a former publicist for the Grateful Dead, is best known for his bestseller “A Long Strange Trip: The Inside History of the Grateful Dead.”

In “The Last Great Dream,” he connects the countercultural dots, chronicling how painters, poets, writers, musicians and other “refugees from the conventional” passed the torch from generation to generation.

He takes a deep dive into countercultural history, but does it with a light touch, breezing through 36 chapters, taking readers from the 1950s Beat poetry scene electrified by Allen Ginsberg’s norm-shattering poem “Howl” and the overarching influence of Jack Kerouac’s novel “On the Road” to the psychedelic touchstones of the San Francisco hippie scene: Golden Gate Park’s Human Be-In in 1967, the Trips Festival and the Acid Tests that launched the Grateful Dead, the ruination of the Haight-Ashbury after the Summer of Love and the lightning in a bottle of the Monterey Pop Festival. There’s a lot more in this fascinating history — London, Greenwich Village and Los Angeles. But most of the action is focused on San Francisco, where McNally has lived for decades.

“Unlike any other American city, San Francisco was not created by sober, bourgeoisie strivers,” he said. “It was created by a bunch of crazies — people who didn’t fit in where they were and came here.”

Marin County, a refuge across the Golden Gate Bridge from the tumult of the city, also plays a significant role in this story.

“There was this constant back and forth between San Francisco and Marin,” said McNally, bringing up Beat poet Gary Snyder, who sought peace and tranquility in a rustic cabin on Montford Avenue in Mill Valley that he dubbed Marin-An, the “An” meaning a lodge or hermitage in Japanese.

Snyder and Kerouac had met at the first reading of “Howl” at poet Kenneth Rexroth’s Six Gallery in San Francisco in 1955. When Kerouac joined Snyder at Marin-An in the spring of the following year, the two Beat icons meditated together and spent many hours hiking on Mount Tamalpais. Before Snyder left to study Buddhism in Japan, he and Kerouac completed a circumambulation of the historically sacred mountain, a ritual that people still do today. That period became the inspiration for Kerouac’s 1958 novel “The Dharma Bums.”

As an indication of how things have changed since then, Snyder rented his shack for $25 a month. Today, a house on that same property is valued at more than $4 million.

In music, McNally chronicles how the Haight-Ashbury rock scene flowed over the bridge to bucolic West Marin in 1966. With Janis Joplin as their new lead singer, Big Brother and the Holding Company got their act together in an old house in Lagunitas. Quicksilver Messenger Service took up residence in Olema, and the Grateful Dead created what McNally describes as “a hippie liberated zone” for six sybaritic weeks at what is now Olompali State Historic Park in Novato. It was a far cry from the band’s previous brief residency in Los Angeles.

“They went from being in South Central Los Angeles to being in paradise,” McNally said. “They had a marvelous time there.”

After they were busted for pot at their famously communal house at 710 Ashbury St. in the Haight in the fall of 1967, members of the Dead escaped to paradise once again, moving their office and themselves to Marin.

“With too many tourists and kids and traffic, the Haight-Ashbury had lost its luster,” McNally said. “The band said Marin looks pretty good, so they all individually moved to different parts of the county. Everybody ended up where they wanted to be.”

In Sausalito, the hedonistic artist Jean Varda lived on the ferryboat Vallejo and became the locus of a scene that included the so-called Sausalito Six, a coterie of young painters from the California School of Fine Arts, later the San Francisco Art Institute, that included Richard Diebenkorn, who would go on to be considered one of the most important American artists of the second half of the 20th century.

“Varda was a traditional bohemian with the wine and the women and the love of life,” McNally said. “Sausalito always had an edge to it.”

McNally was inspired to write this book after he was invited by the California Historical Society in 2016 to curate a photo show celebrating the 50th anniversary of the Summer of Love.

Working on that project, he could see how Beat poets like Ginsberg and Michael McClure were the forerunners of Bob Dylan and other poetic rockers.

“There are a couple of moments in the book when that whole process is obvious,” McNally said, “and that’s one of them.”

Another example is the influence of Beat artist Wally Hedrick on a teenage art student named Jerry Garcia, who took a painting class from Hedrick at the San Francisco Art Institute.

“Wally was very consciously Beat, anti-war, anti-consumerism, anti-success and anti-fame,” McNally said. “Very much not your average guy.”

During the class, Hedrick played bluesman Big Bill Broonzy and other folk music as the students painted. He also turned Garcia on to Kerouac’s “On the Road.” Both those things, the music and the book, would have a profound influence on the fledgling guitarist.

“‘On the Road’ was Jerry’s bible for the rest of his life,” McNally said. “It set him on his path. And after that, any room with a guitar would be fully furnished.”

The values that Hedrick imparted would also mold Garcia’s career as a musician and artist.

“Being focused more on art and love and less on money and achievement were what was important,” McNally said. “Jerry’s later commercial success shocked the hell out of him and was never intentional. It was just a matter of luck.”

For McNally, “The Last Great Dream” finishes a lifetime of writing about bohemia in America, beginning with “Desolate Angel” about Jack Kerouac, then his bio of the Grateful Dead, followed by “On Highway 61,” about the relationship of White people to Black music.

In this look back at the hippies and the cultural forces that created them, he shows how the ‘60s still resonate today. What was once radical has become mainstream. He sees this clearly whenever he talks to young people about those times.

“Kids are interested in this because it’s still absolutely relevant to their daily life,” he said. “Values about sexuality, drugs, expanding consciousness, music — it’s all still meaningful.”

Contact Paul Liberatore at p.liberatore@comcast.net