The Beatles, a double album by the band of the same name.
The album cover is completely white.
After the heavily orchestrated and produced Sgt. Pepper's Lonely Hearts Club Band album, the Beatles return to simpler arrangements, mostly played on house&garden variety instruments.
This change can be explained in several ways.
Personal reasons: the band members needed to break away from the relentless pace and discipline that had made the Beatles so successful, and develop personal lives outside of the band. Obviously this took its toll on the amount of energy spent on the music. As a matter of fact, they found it difficult at times to be in the same recording studio at the same time. Most of the material in this period was done with only 2 or 3 Beatles at once, with other members absent or overdubbing their contributions later.
From a musical viewpoint, the Beatles had probably pushed orchestration and overproduction to its limits. Songs like "Strawberry Fields Forever", "Being for the Benefit of Mr. Kite", or "A day in the life", exist only on record; they would fall apart if you took them off and tried to perform them live. By contrast, the songs on the White Album can be readily performed in a pub or at a camp-fire.
What this means today, at least for me, is that the world famous Sgt. Pepper turns out to be an interesting, but overpretentious and sometimes unbearably heavy album, while its successor is as fresh as it ever was. It doesn't hurt to be unpretentious when you excel at it.
True mastery is the level of skill where everything seems to come naturally and effortlessly; to me, this is the album where the Beatles reached that stage. They didn't stop trying to improve: the album is full of new and unexpected details, and most of the songs are arranged and played with the usual care.
In the arrangements, for example, you can hear: an airplane (to open and close "Back in the USSR"), an old-fashioned harpsichord (in "Piggies"), a crazy Spanish guitar riff (in front of "Bungalow Bill"), an off-beat riff to confuse the hearer ("Everybody's got something to hide except for me and my monkey" - compare the one on "I don't live today", which actually confused the singer!), a bass part by voice (I will), a "broken piano" and "drunken violin" ("Don't pass me by"), and so on.
Most of the up-tempo "fun" songs are McCartney's.
"Back in the USSR" is a Beach Boys parody, although -- I owe this to a wu below -- not the one McCartney wrote as a birthday present for their singer, Mike Love.
Ob-la-di ob-la-da and Honey pie are songs in the music hall tradition McCartney grew up with. But there are less annoying "fun" songs all over the album: "Piggies", "Everybody's got something to hide except for me and my monkey", "Wild honey pie", "Bungalow Bill".
The melancholic songs are the ones that stick: "While Eric Clapton's guitar gently weeps", "Sexy Sadie", "I'm so tired", "Don't pass me by", "Blackbird".
And then there is "Helter Skelter".
(I could go on about this music, but there's a great site on it already:)
http://www.tacoshell.com/spinalcracker/cgi-bin/search.pl?index