People who came of age during the 1960s may find it hard to believe, but this weekend marks the 40th anniversary of the release of Sgt.Pepper's Lonely Hearts Club Band. Many critics have said of the Beatles' groundbreaking album that it epitomizes the best of 60s popular music, and still ranks among the finest music of any kind of the 20th century. Four decades later, it still sells 5000 copies a month worldwide.

For young people at the beginning of 1967's so-called "Summer of Love," Sgt. Pepper's Lonely Hearts Club Band was a complex and joyous musical expression of the revolutionary changes underway in popular culture and politics. "People were just breathless about it," says author and music critic John Rockwell. "It was like (the) Harry Potter (book series) except for grownups rather than little kids?!"

And like Harry Potter, today's fictional boy wizard, Rockwell says he and other young adults in the 60s were dazzled by the Beatles' bold musical experiment, even amidst other great contemporaries such as singer-songwriter Bob Dylan, jazz trumpeter Miles Davis, and Indian sitarist Ravi Shankar.

Rockwell says there was a feeling that there was "a new openness" in music with different genres coming together in interesting ways. "That was the sensibility which just pops off the grooves of Sgt. Pepper."

Its 13 songs ranged from the gently ironic of "Getting Better"? to British urban pop of "Lovely Rita, Meter Maid"? to the mystical "Within You Without You"? to the sentimental and nearly operatic "She's Leaving Home."

The sophistication and range of the album prompted many in the highbrow classical music world, including the famed American composer and conductor Leonard Bernstein, to praise Sgt. Pepper for its unusual blend as "classical music."

But the notion of Beatles songs as classical music didn't sit well with many younger people who were more at home with the fun, upbeat hits the band had released during the early 1960s. Today, Marshall Crenshaw is a rock musician and composer in his own right, but when he first heard haunting and mature songs like "Day in the Life" as a young teen, he says he missed the four "lovable, laughable, madcap mop-tops from Liverpool" he once knew.

"I was frightened by it at first. There is this sense of joy mixed with terror," Crenshaw says. "I wasn't ready for it. It just sounded otherworldly. It was in this imaginative realm that was so alien compared to the earlier music that I was used to."

But as time went by, Crenshaw says he played the Sgt. Pepper album, and he "fell in love with it, because in the first place it rocked, which surprised me!"

Crenshaw says he was drawn into the album by the song "Good Morning, Good Morning," not only because it rocks, but also for its "cinematic" lyrics. "The scene changes in your mind as you go along," he says, and recalls when he realized "this is rock music, just a different kind of rock music!'"

The imagery in several Sgt. Pepper songs sounded more like visionary poetry than traditional rock lyrics. And critic John Rockwell says that the tape loops and experimental techniques that producer George Martin employed in the recording studio added to the impression that this was truly a new kind of music.

"The whole album is distorted in various ways," Rockwell says. "Every single note is processed or filtered or overdubbed or altered. So the whole thing is like a funhouse mirror, aurally speaking."

The funhouse metaphor is accurate. For "The Benefit of Mr. Kite," a John Lennon song inspired by an old-fashioned traveling carnival poster, producer George Martin tape-recorded old calliopes, steam organs and other fairground sounds, snipped the audio tape into half-meter lengths, mixed them up and spliced them back together, often backwards.

Brooklynite Beatle fan Mario Sinisi didn't know how it was done but says he didn't care. "We smoked grass and we used psychedelics, and listening to Sgt. Pepper was what a good trip was about!"

Indeed, according to music critic John Rockwell, it is impossible to separate Sgt. Pepper's psychedelic music from the mind-altering drugs the Beatles used and that many fans used while listening to the record. But Rockwell says that for everyone who immersed themselves in the Beatles' new music, there was a sense of being a part of an important artistic and social vanguard, a "new generation."

Rockwell concedes that belief was naïve, but adds, "the majority of us, we went on to lead plausibly productive lives but with a transformation of our sensibilities, a transformation I would argue was entirely positive, 99 percent positive!"

That transformation seems to be ongoing. As one youth at Central Park's Strawberry Fields memorial to John Lennon told me on the eve of Sgt. Pepper's 40th anniversary, "Every generation discovers the music in its own way."