I am not sure about the wisdom of calling a tribute album to John Martyn, Johnny Boy Would Love This, but fair enough, maybe he would. Or maybe he would have liked some, maybe not others. Or maybe he would have raved about it and shouted it from the rooftops. Anyway, the discussion is academic, because what the late musical genius who died in January 2009, at the age of 60 would make of it is, when it comes down to it, immaterial. No offence, guv.

These two records which amount to well over two hours of music, make for a carefully-constructed labour of love. The project was conducted under the stewardship of Chicago-based record producer Jim Tullio and Pete Mason who began almost immediately following John's death. Tullio, along with engineer Gary Pollitt, handled the exhaustive work involved in completing Martyn's posthumous album Heaven and Earth, which also appeared in recent months. Tullio and Mason started contacting various artists about the idea of putting together the tribute. California-based label manager Jim Snowden came on board about 15 months ago, overseeing distribution and marketing on a worldwide basis.

Tullio steps into the arena himself and sings Martyn's Road to Ruin - and damn good it is too, as rendered here, earthy and robust. Some of the tributes were recorded when John was alive. Indeed the man himself played guitar on Brendan Campbell's version of Anna, and on Cheryl Wilson's fine interpretation of You Can Discover. That's him saying "my fault entirely" as the latter song begins - he fluffs the chords, but even John Martyn's fluffs were interesting. He mumbles some mild approval too when Cheryl, who has a wonderful voice, has stopped singing.

Somehow you feel the apprentice's love for the master which all the artists concerned have for Martyn's lengthy 40-year catalogue of glorious songs. These artists' valiant, sure-footed - and mostly successful - efforts to rise with musical elegance and grace to the challenges concerned is profoundly touching. Here you have assembled not just great singers, but incredible musicianship.

The Cure's Robert Smith adeptly handles one of the most difficult-to-get-right pieces on the record, Small Hours. Although, Smith's version is busier and more complex than Martyn's, it takes the necessary risk and finds a new brightness, a new effulgence, a new power in the song. Smith first heard Small Hours on The John Peel Show in 1977 and One World, the album from which it is taken, happens to be his favourite.

I felt beforehand, when I had heard one or two tracks in advance, that Judie Tzuke's version of the lovelorn, lovetorn ballad that is Hurt in Your Heart would be the best thing on this double-CD record. It's all subjective of course, but this will probably in fact be the case for many fans.

Now a mature woman in her mid-fifties, Judie was the shy young girl who had a hit with Stay With Me Till Dawn back in 1979. The singer brings a touching vulnerability to one of Martyn's best songs. She makes the song almost entirely her own - talk about a perfect fit.

But others will incline more to the husky, ghostly take on Couldn't Love You More from Lisa Hannigan, accompanying herself solely on the zither. This is a spine-tingling and definite highlight, just give it a listen. Beck - a long- time Martyn fan, it seesm - also brings a ragged beauty and almost a sense of ownership to Stormbringer.

Phil Collins' version of John's Tearing and Breaking was completely re-recorded since its first appearance on Love Stories, the Collins love songs compilation, released some years ago by EMI.
A somewhat lesser-known act, The Blackships (featuring David McKellar) address a correspondingly lesser-known song Rope Soul'd. Swaying and mantra-like, Rope Soul'd was a subtle sketch, almost an afterthought on the album 1984 album Sapphire. In some 20 Martyn performances witnessed by this writer, I never once heard him perform the song, one of his most interesting creations. The Blackships fill out Rope Soul'd, taking it into trippy, mild electronica.

Vashti Bunyan's rendering of Head and Heart is beautifully ethereal. The band Oh My God stick close to the power horse that is John Wayne, but bring freshness too, as does Brendan Campbell on Anna. Fine Lines enjoys an Afro-Caribbean makeover from the recently deceased Syd Kitchen.

Snow Patrol bring something smart and intriguing to May You Never, by slowing it down and injecting a kind of drama. But after a while it palls a little - by the time John had passed on, many of us had heard that song perhaps too often for its appeal to be easily discerned. Yet when you hear it for the first time, it’s an entirely different story and if Snow Patrol are the conduit for people to hear about Martyn, then all to the good.

Swell Season (Glen Hansard and Marketa Inglova) do a good version of I Dont Want to Know, but like May you Never, the choice is one of the short straws. The same applies to Paolo Nutini and his version of One World and Skye Edwards, formerly of Morcheeba, who sings Solid Air. These numbers are too strong already, too definitively honed and done in different arrangements by their writer down through decades of performance.

Clarence Fountain and Sam Butler, formerly of the Blind Boys of Alabama, disappoint a little with Glorious Fool , although this is purely personal and word has it that it has gone down very well with folks who have heard this record in advance. It should be said that the song was often the strongest number in a latter-day Martyn and band set. More of the layered vocals that characterised The Blind Boys sound might have helped. The magic of Martyn's late performances of Glorious Fool - swaggering, angry, growlingly bluesy - were enhanced by adventurous chord phrasings from his long-time sideman, Spencer Cozens. Cozens' keyboard mastery lent a searching, intrepid dimension that pulled the song away from its shuffling blues template.

Beth Orton, on the other hand, does a richer version of Go Down Easy than John himself did when he rearranged it - unwisely I would argue - for his 2004 album On the Cobbles. David Gray's version of Let the Good Things Come is perfectly fine, but somehow I can hear James Taylor performing the song with equal empathy.

On the other hand, you get some of that vital black vocalising and layered vocals I referred to earlier on the vibrant Dancing, performed here by Sonia Dada. This is a new name to me, amongst a host of new names. Irish musician Ultan Conlon's version of Back to Stay, the best song on John's first album, London Conversation (released in 1968) is so touching it may summon your tears. John, I know would definitely love this one, no contest. And he couldn not deny similar pathos in Bombay Bicycle Club's deeply touching take on Fairytale Lullaby. Morcheeba (featuring Bradley Burgess) tamper little with the early song Run, Honey, Run and skillfully flesh out its acoustic properties,without making it into something blandly rockist. Quite aside from everything else it does, this record does a great favour by revitalising the often overlooked early Martyn songs.

In my time, I have often fantasised about singers doing Martyn versions. I particularly want to hear Van Morrison's take on Lookin' On, if it could be arranged. Or one muses about one of the great black singers from this record doing Our Love from the Grace and Danger album. Or Phil Collins having a go at Hold on My Heart; Barry Gibb doing Lonely Love, Tony Hadley giving some further zip to You Might Need a Man. I'd like to hear Michael MacDonald singing Back with a Vengeance, Massive Attack retreading the pitless, relentlessly rolling thing that is Amsterdam. There is no doubt that Don't You Go would be beautifully sung and played on guitar by Dick Gaughan .

Or think of Steve Winwood having a crack at Sweet Little Mystery, Could've Been Me or Fisherman's Dream, Beth Gibbons or Tony Bennett doing So Much in Love With You. The Rolling Stones essaying Grace and Danger, or Gambler from the recent Heaven and Earth, Peter Gabriel singing Sapphire. Or take Mark Knopfler warbling his way through Mad Dog Days. Or what about bold new versions from a selection of artists of all nine songs from the posthumous album Heaven and Earth. In its more freewheeling, rough-at-the edges moments, that record somehow suggests rich possibilities of further improvisations on themes. Could Have Told You Before I Met You, for instance, seems tailor made for Paddy McAloon and the song already has something of that early Prefab Sprout sheen about it for starters.

The final package of this Martyn treasure trove will also include a 40-page booklet, a DVD featuring interviews with some of the artists concerned, performance videos, and even some rare John Martyn performances. All proceeds from the sale of the record will go to Martyn's family and the record will be released on August 12. You can pre-order on iTunes from July 16.

Paddy Kehoe