To attempt to offer any kind of detached appraisal of this album would be farcical: 1959's Kind of Blue is as much a part of the fabric of modern music as Sgt. Pepper's or Thriller. If you ignore Kenny G's success (and you should), it's the best-selling jazz album of all time. Boasting five tunes that are as iconic and instantly recognizable as anything produced by the great pop groups of the 1960s, Miles Davis' landmark recording has weathered extremely well. Indeed, the older it gets, it seems to stay the same age.

So what new is there to say about this post-bebop masterpiece now, half a century on? Quite a lot, actually. Listening to the album now still turns up nuances within the performances previously not heard, new threads in the harmonic web. The open-ended possibilities of early modal jazz forced really these six top players to think on their feet -- this style, in which Miles and his group had begun working a few years prior on Milestones, requires players to work from scales (modes) when improvising, rather than simply playing over chord changes. So the melodies, such as they were, came free-flowing from the minds and hands and mouths of the musicians. That such an enduring work was produced almost entirely on-the-fly continues to boggle the mind.

Of course it helps that is one of the greatest combos ever recorded: Julian "Cannonball" Adderley on alto, John Coltrane on tenor, Bill Evans and Wynton Kelly at piano, Paul Chambers on bass, and Jimmy Cobb on traps. The chemistry of the personalities -- Adderley's studied chops against Coltrane's manic, cascading flourishes, with Miles dulcet horn calling the two together -- certainly makes for an exciting listen, even on the 20th or 30th listen. And the smooth simplicity of the themes, which were composed entirely by Davis (though Evans is widely credited with pulling the arrangements into focus), allows the players to stretch out and explore.

The new Legacy edition reissue of Kind of Blue (due out Jan. 20) is a completist's dream: The two discs nearly triple the album’s running time. The first augments the original group of tunes with alternate takes, false starts, and entertaining studio banter from the March and April 1959 sessions. It's far from essential, but it's fun to hear Miles' back-and-forth with the recording engineer.

The second disc delivers the goods: the only other studio cuts studio tracks cut by the Davis-Adderley-Coltrane-Evans-Chambers-Cobb group. These four tracks (and one alternate take), recorded a year prior to the Kind of Blue sessions, have been released publicly before, but they're included to provide perspective on the genesis of Davis' band. Adderley opens up more on these tunes, particularly the expansive take of "Love For Sale," and "On Green Dolphin Street" finds the rhythm section in particularly playful form. The set is capped by the first authorized release of a speedy, 17-minute "So What," recorded at a concert in Holland in April 1960. The group on this date -- without Adderley, Kelly having replaced Evans on piano -- drops the atmospheric intro from the recorded take, getting right to the meat of the tune. They sound lean and aggressive, with Kelly's lively playing a particular highlight, and Davis' misleadingly mournful long notes in the closing moments sounding more like a beginning than an ending, like the tune is turned on its head. It's a hell of a performance.

Legacy is also offering a box-set version of this set that includes a new vinyl pressing, a DVD, and other memorabilia, though the price point dictates that it's not for the casual Miles fan (it retails for more than $100). Even the two-disc reissue is a trifle, really; just another way to draw attention to an album that's never needed any help with such things. At age 50, Kind of Blue truly maintains its cool.