It is in some way appropriate, given the murky waters of humanity that Gilbert and George's own work often explores, that the hunt for the book has now turned to a ghoulish finger-tip search through the dead man's belongings in his Devon country cottage.
Farson was still knitting together the biography, provisionally entitled The Extraordinary World of Gilbert and George, when he died unexpectedly after a bout of pneumonia.
Gilbert and George admit they have no idea where the manuscript, full of precious and previously unpublished anecdotes, might be.
Farson, former TV presenter, talented writer and photographer and notorious Soho drunk, seems to have taken the whereabouts of the manuscript to the grave: no one knows where it is.
HarperCollins commissioned the biography, which was due for completion next month and, according to his publishers, Farson always managed to meet his deadline - despite a penchant for heavy drinking.
Farson, the son of a celebrated war reporter Negley Farson, was a promiscuous homosexual who never married and left no close family. His previous works include Gilbert and George in Moscow and Limehouse Days, A Personal Experience of the East End. His life story, Never A Normal Man, sold out after it was published last year.
His other biographies include one of the author of Dracula, Bram Stoker, to whom he was related, and another on his friend of 40 years, the artist Francis Bacon. The best-selling volume The Gilded Gutter Life of Francis Bacon (1993), originally commissioned in 1982 but delivered 10 years later - after Bacon's death - was seen as Farson's greatest achievement. The subjective biography is full of Farson, his life blended with that of his subject. His 1991 book on Gilbert and George in Moscow had a similar agenda.
Farson's editor at Harper Collins, Richard Johnson, and his literary agent, Bill Hamilton, travel to his Devon cottage this week to search for the manuscript and other material Farson is thought to have collected for the book. "It will be very spooky to go through a dead man's life in this way," said Mr Hamilton. "But we are determined to find the manuscript. None of us have seen it, but we are convinced Dan has left it in the cottage. We are expecting a tome of about 100,000 words." Gilbert and George have won world acclaim with such works as their Flying Shit series of pictures. They won the Turner Prize in 1996 and, despite heavy criticism in some circles, their work remains obsessed with male nudity and excrement. They recently produced a Gilbert and George computer screen-saver, sold in aid of an Aids charity, Crusaid.
The pair, who have lived and worked together at their 18th-century home in Spitalfields, east London, for 30 years, rarely give interviews but, talking jointly - as they always do - they have now expressed their fears about the future of the book.
"We don't even know for sure where the manuscript is. We are obviously curious and just a little concerned. This has been a huge investment in time and love for us and we trusted Dan to do it properly. Since then we have heard nothing from anybody.
"He was a brilliant writer and an extremely honourable individual. He came with us to New York, Japan and Stockholm and collected lots of anecdotes and the story of our lives. We don't know what is going to happen with all his work."
The pair had been asked several times to co-operate with a biography, but resisted until approached by Farson, a friend.
Now, the search turns to the dead man's cottage in Appledore, Devon, which is being looked after by a housekeeper. Mr Hamilton and Mr Johnson have permission from the estate of Dan Farson to hunt through his belongings.
They will first have a private meeting with Mr Farson's solicitor. The estate is still being wound up and an estimate for the market value of the yet unpublished biography is expected to be made next month.
It is all, as Mr Hamilton confirmed, a delicate matter. "I would say Gilbert and George have nothing to worry about but yes, we do not know what we are going to find.
"One thing for sure is that despite his boozing, Dan churned out nearly 30 books. Although there will be lots of secrets, you could rely on Dan to be extraordinarily discreet about his work and what he knew about the artists.
"It will be a relief to find it and the plan is to bring it back to London. I will ask Gilbert and George to help come up with a title, but it will not be published now until next spring."
Gilbert and George guard both their private lives and the sale of their work zealously. Four pictures were recently sold in New York for pounds 100,000 each, and they rarely give interviews or co-operate with books unless they have complete editorial control.