Word of mouth, a phrase that first crops up in Twelfth Night, is the holy grail of book publishing. Good reviews and literary festivals are all very well. But with the wind of many voices in its sails, a book can reach millions. Word of mouth made Dan Brown and JK Rowling. As the world's publishers fly into London for the book fair tomorrow, word of mouth will be what they crave for their titles.

Some of them will go to any lengths to stimulate the phenomenon. One leftwing 1930s publisher, Victor Gollancz, used to bribe his staff to read his books on the Underground. No one could miss their lurid yellow covers. Perhaps a casual commuter conversation would inspire sales mania.

With the advent of social networking, word of mouth begins to enter the realm of science, at least in theory. Actually, despite Twitter, Facebook and the rest, publishers are finding it as difficult as ever to mobilise that elusive thing, the viral conversation about a new book that translates into worldwide sales.

That doesn't stop them from trying. John Murray is about to publish James Frey's The Final Testament of the Holy Bible, a transgressive version of the Gospels. Frey, a seasoned contrarian, is good at attracting attention, not necessarily of the right kind. Ever since he was exposed for passing off his fiction A Million Little Pieces as a misery memoir, he has been the object of controversy.

Murray has cleverly decided to exploit this with a YouTube video of vox pop interviews about The Final Testament. Murray's MD, Roland Phillips, says: "We are getting into a new arena. We hope to inspire word of mouth. It's the best way to sell books, especially today. People don't like to be told what to read, but to make their own discoveries."

What, exactly, is this mysterious phenomenon? A comforting short -hand to explain the inexplicable? Dava Sobels's Longitude got no more, or better, reviews than several other books in 1995, but sold like hot cakes. Tony Parsons's Man and Boy got a mixed press, but went straight to the top of the bestseller list. Is it just readers on trains and buses, or at dinner tables, telling their neighbour, "You must read this" that does it ?

I don't think so. The key to word of mouth lies in the milieu into which the unknown book is released. Appropriately, the classic case is The Tipping Point by Malcolm Gladwell, a book about the little things that make a difference in the marketplace. When it first appeared in 2000, The Tipping Point excited only modest comment. The reviews were scattered and lukewarm. It was not until Gladwell, an entrancing speaker, went out on the road to talk to professional groups across America that The Tipping Point "tipped".

In a practical sense, Gladwell created his own word of mouth. To generate a surprise bestseller, the publisher needs to create a community around the book and its author in which the readers believe that their aspirations can be understood, their beliefs nurtured and their anxieties addressed. That's what a successful book will do.

This helps to explain two word-of-mouth phenomena, Divine Secrets of the Ya-Ya Sisterhood by Rebecca Wells and Eats, Shoots & Leaves by Lynne Truss. Ya-Ya Sisterhood first popped up on a bestseller list in northern California. In the days when such things were a novelty, it was a "book-group book". Then readers started to come to Wells's readings. They didn't just buy the book for themselves, they bought it for their family and friends. Word about Ya-Ya Sisterhood spread nationwide. Wells spent a year on book tour, and sold millions.

Eats, Shoots and Leaves, a global bestseller, appealed to a much larger, but more informal community: anyone who felt that the world is going to hell in a handcart and that, more to the point, believed that all their fears could be illustrated by the decline of English usage. Truss said her book was a "zero tolerance approach to punctuation". But it wasn't really about grammar and punctuation, it was about bourgeois fear. Word of mouth: the marriage of enthusiasm and anxiety.

The literary world will miss you, Christopher

Christopher Sinclair-Stevenson has long been one of the timeless figures on the London literary scene. He made his debut as an author in the 1970s with Inglorious Rebellion, wrote an admired book on Anglo-French relations (That Sweet Enemy); and subsequently, as an editor, published the early works of Paul Theroux, Jane Gardam, AN Wilson, Peter Ackroyd, and William Boyd. Lately he has been a publisher and literary agent. His eponymous imprint is now bowing out with the recent launch of Giles St Aubyn's impressive study of Victorian faith, Souls in Torment, but he will continue as an old-style man of letters who refuses to be pigeonholed. According to one literary website "there are no categories or genres listed for this agent". I wish him well in his busy retirement.

From the art of fiction to a much broader canvas

Reg Gadney, the acclaimed author of Just When We Are Safest and several Alan Rosslyn thrillers, started his literary career writing about Constable and has a second, more private reputation as an artist. Gadney usually keeps his artistic side well hidden and disdains bohemian ostentation. "I like the idea of being a bourgeois painter in a suit," he says, "like Magritte." He hasn't published a thriller since 2003 (The Scholar of Extortion), but an exhibition, Reg Gadney: New Landscapes, has just opened at Celia Lendis Contemporary in Moreton-in-Marsh, Gloucestershire. The collection is a powerful celebration of solitude and silence. "When I breathe in," says Gadney, quoting a Japanese poet, "there is a sound in my body sadder than the winter wind." Exactly.