Drawing on archives, demos and unreleased or hitherto discarded material, so many more have appeared since her untimely death, from a brain haemorrhage a month after a household fall in 1978, that it may have been thought the barrel had been scraped clean.
Not so. In what we can surely take to be one last burst of cultural archaeology, Universal - as the parent company of the Island label - has unearthed enough unheard music and rare interview tapes to justify a 19-CD box set intended to cover every note and every word known to have been sung, played or uttered in her professional life.
Denny was a woman with a powerful personality but moments of great vulnerability, and her rare vocal quality benefited from both the strength and the occasional frailty. She desperately wanted to be a successful solo artist, and left evidence of sound achievements in that aspect of her career. But this assembled collection confirms that it was when fronting bands, briefly with Fotheringay but most of all in her stints with Fairport, that she truly excelled.
A boxed set of this nature does not, indeed could not, come cheap. Even at Amazon, it comes in at just under £150; there is a special edition, with a framed, signed print of Phil Smee's artwork, that costs even more.
Fans who already possess a number of the original albums and posthumous releases have raised eyebrows at these prices. Even so, there is no dispute that the clout of Universal has combined with Island's history to produce a seriously important treasure.
There are 16 previously unreleased songs, among them a convincing interpretation of Bob Dylan's It Ain't Me Babe, among more than 100 alternative versions of tracks that already form part of the Sandy Denny discography. Richard Thompson buffs will enjoy a studio take of the familiar Blackwaterside, with their hero (also Denny's musical guru) working out the arrangement as they go along. I loved hearing such rarities as the wonderful Jackson C Frank song Blues Run the Game, and other ingredients from the staple diet of the folk clubs from which Denny sprang to become a revered figurehead of folk-rock.
From time to time, Denny liked to indulge a fondness for out-and-out rock and pop. Two examples, Memphis Tennessee and That'll Be The Day, may not impress Chuck Berry and Buddy Holly aficionados but, live or in the studio, she seemed to get a lot of fun out of singing them.
The documentary package has everything from the commendably thorough - a 72-page hardback and a notebook with Denny's handwritten drafts of her songs' lyrics - to pure trivia (a copy of the receipt for her first piano). And it amounts, beyond question, to a distinguished collector's item.
Every other artist who manages to hold down a career for a few years seems these days to qualify for a box set. Hardly any performer warrants quite this treatment. But then Sandy Denny was hardly just any performer.
Related listening (The National's choice):
Fairport Convention - Liege and Lief (1969)
Denny's extraordinary vocals resonate throughout Fairport Convention's fourth and finest album, which saw the band reach back into the British folk songbook (only two of the eight tracks are originals), literally electrifying their interpretation of traditional standards to produce a classic of the genre whose influence is still felt today.
Jackson C Frank - Blues Run the Game (2003)
A former boyfriend of Denny's, Frank was a familiar figure on the early 1960s folk scene. Although his songs were covered by Simon and Garfunkel and Nick Drake, he remained a troubled, enigmatic figure. Rediscovered shortly before his death in 1999, this two-disc compilation is a definitive collection of his work.
Smoke Fairies - Through Low Light and Trees (2010)
As their vocal style attests, this acclaimed English duo, Katherine Blamire and Jessica D Davies, also cite Denny as a major influence, their moody, ethereal, atmospheric sounds tinged with touches of traditional British folk and American Deep South blues.
Related listening (my choice):
Fairport Convention: History of … (Island)
Not everyone's budget runs to lavish box sets. But there is plenty, from reissued albums to less ambitious collections, to satisfy prudent buyers. It is difficult to fault this compilation covering Sandy Denny's first spell with Fairport Convention. Most classics - including Who Knows Where the Time Goes? at its best - are present, and the vocals are pure magic.
Sandy Denny: No More Sad Refrains (A&M / Island)
A double album that accompanied Clinton Heyman's biography and borrowed from its title. It assembles material from Denny's work in bands, including important Fairport and Fotheringay moments, and from solo efforts including The North Star Grassman and the Ravens (1971) and Rendezvous (1977).
Sandy Denny Boxful of Treasures (Fledg'ling)
So the compilations do not go far enough, but you still cannot stretch to a 19-CD collection. Meet the slim ancestor: five albums, a book running to just 56 pages (though with a Richard Thompson foreword) and unreleased work ranging from home-recorded demos to Knockin' on Heaven's Door, with Fairport, live in Los Angeles in 1974.