There are few albums in rock 'n' roll history as sacred and as legendary as the BeatlesSgt. Pepper's Lonely Hearts Club Band.

It's been picked apart and written about to death, with every note of its nearly 40 minutes scrutinized. From the classic double-hit single that started the album's sessions to the final sustained note that ends the LP, there isn't much left of the Beatles' most celebrated record that the group's most devoted fans don't know.

Even the countless hours of studio time that went into the making of the album have been documented on various bootlegs and even an official release in the 50 years since Sgt. Pepper's release. With the six-disc Sgt. Pepper’s Lonely Hearts Club Band: Anniversary Edition, the Beatles aim to have the final word on the matter.

It's an impressive set. Maybe not as extensive as a similar collection assembled by one of the Beatles' main competitors, the Beach Boys, whose Pet Sounds sessions box collected hours of alternate takes, studio chatter and false starts, but the breadth of material collected on the four audio CDs (the other two discs contain new surround mixes and video programming) is astounding: a 2017 stereo remix, the original mono mixes of the album and the "Strawberry Fields Forever" and "Penny Lane" and, best of all, more than 30 tracks from the sessions that chart the growth of the album's songs from near demos to near completion.

The most revealing cuts fall somewhere in between: the five takes of "Strawberry Fields Forever" that chart its evolution; the first stab at "A Day in the Life," with the famous final chord hummed rather than played on pianos; the earliest take of "Lucy in the Sky With Diamonds"; and an instrumental version of "Within You Without You" featuring various Indian instruments.

At times, the Anniversary Edition drops listeners right into the studio with the group. Hearing songs -- particularly "Strawberry Fields Forever" and "A Day in the Life," but also highlights like "Penny Lane" and "Being for the Benefit of Mr. Kite!" -- take shape as the Beatles and producer George Martin work them out is often fascinating, though it does become a bit tedious at times, especially when certain songs didn't change much from inception to release.

The sessions are the heart of the box, but Giles Martin's new mixes are eye-opening too. He worked with his father's original tapes, and the new remix he gives the finished album combines the punchier mono version from 1967 with state-of-the-art tweaking that brings out every rattle and breath buried in the songs. It gives new life to one of the most popular records ever made. (This new mix is also available on single- and double-disc editions.)

Sgt. Pepper fanatics will get more out of all this than casual Beatles listeners. Even so, 50 years after its release, the album remains a landmark recording and document of the era. It's a timeless cultural marker that hasn't lost much of its ability to dazzle after all these years. And from the sound of things, Sgt. Pepper will never go out of style.

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