Literary agents are the gatekeepers and tastemakers of the book world, deciding an unpublished manuscript's fate even before a publisher does. No wonder then that Mr Jonny Geller, joint chief executive officer of the Curtis Brown agency, was last year named among Britain's 500 most influential people by long-established society watcher Debretts.

For publishers, agents act like "a filter for the thousands and thousands of manuscripts floating around", says Mr Geller, whose agency represents authors from spy thriller writer John le Carre to satirist Howard Jacobson and historical novelist Tracy Chevalier.

He has been with Curtis Brown since 1995, starting as an assistant to an agent because his acting career had failed to take off. The person he was assisting left the agency and left behind a manuscript about a psychopath's revenge on his past teachers. Alerted to the story by a phone call from the first-time author, Martyn Bedford - now an awardwinning writer of multiple novels - Mr Geller read it, loved it and set off an auction between two publishers. Acts Of Revision was eventually published by Bantam Press in 1996.

If the sale had been unsuccessful, Mr Geller could have lost his job for exceeding his remit. Instead, he became a full-fledged agent.

In a telephone interview with The Sunday Times from his London office, he laughs, recalling the excitement of that first sale. "The huge pleasure that an agent gets from behind the scenes is greater than what I found as an actor, in front of an audience."

Then there is the joy of helping a writer polish his work. "To be side by side strategising with John le Carre is an enormous privilege. It compensates for not being in the limelight," he says.

An agent not only manages an author's copyright, he is also often a counsellor, editor and friend to the writer. "The literary agent is the objective friend of the writer, the only one in the system. Although publishers care for authors, there's no question that the commercial imperative of the publisher is to sell the books.

"I don't think anyone in the creative industry is doing it for altruistic reasons," he adds. Agents invest their time and are rewarded if the author benefits by a book deal.

Gone are the days when agents received huge envelopes stuffed hopefully with typewritten sheets. Curtis Brown's submission portal is online and that has increased the number of submissions to sometimes thousands a week.

Agents in India and Hong Kong report dozens or more submissions daily. It has been a tough job convincing the industry in those territories of the benefits of working with an agent.

Ms Marysia Juszczakiewicz (pronounced yush-cha-ke-vich) has been a literary agent in Hong Kong since 2007 and set up her own Peony Literary Agency in 2010 to bring voices from China to a wider audience. When she started, she says: "Publishers wanted to deal directly with the author because that was how it worked historically in the Chinese book industry. Authors were tentative about dealing with an agent because they didn't want to damage relations with the publisher."

The first author she signed was Su Tong, whose novella was turned into the 1993 film Raise The Red Lantern by director Zhang Yimou. She was also the first to represent English rights for Mo Yan, the Nobel laureate for literature now signed with The Wylie Agency in the United States.

Today, she represents celebrity novelist and race-car driver Han Han - a translation of his essays will be published by Simon & Schuster later this year - as well as the sword-fighting novels of Wang Du Lu, which inspired the Crouching Tiger, Hidden Dragon movie in 2000 (a sequel produced by Netflix will be released next month).

Ms Jayapriya Vasudevan, who founded the Jacaranda literary agency in 1997, recently negotiated a $20,000 deal with Penguin India for Singapore-based novelist Krishna Udayasankar. She gave critically acclaimed Indian writer Anita Nair her start, selling her debut, A Better Man, to Penguin India, and also represents Singaporean author Suchen Christine Lim.

However, when she started in India in the late 1990s, she was regularly rebuffed during attempts to shop manuscripts. "I guess publishers were dismissive since they had no idea what an agent did. Especially one in India. It took them a while to understand the business and to trust me. That really is the core of the agency business, trust."

Publishers must trust the agent's taste and writers also have to trust the agent to do his job.

Many writers are surprised at how long it might take for their manuscript to be accepted by a publisher, let alone printed, says Ms Vasudevan. A book about being bipolar remained on her list unsold for eight years until last year. Another writer's book has been submitted 74 times without finding a home.

"It can take a year or three for an editor to read your book," she says.

Big publishers may be becoming more risk-averse as well. Mr Geller says. "At the moment, we're going through a phase where experimental or original novels are coming out from independent publishers." That worries readers like him who are always on the lookout for originality, instead of cookie-cutter bestsellers.

Even if a book goes from author to agent to publisher in record time, as in the case of Bedford's Acts Of Revision, that is no guarantee of immediate success. The day the novel was set to reach bookstores, Mr Geller says, the publisher called him with bad news. A gunman had entered a school and fired on students. "Selling a book about a student's revenge on his teachers was impossible. It was quite a sobering story."

In spite of all the odds against new writers, Curtis Brown welcomes unsolicited submissions via its online portal, which can get thousands of entries a week. "The only important thing an agent does is working with the new writer," says Mr Geller, bemoaning the fact that his managerial duties at the agency leave him less time to nurture new voices.

"The only thing we have stopping us is our inability to read them (the submissions)," he says. "If the writers don't have connections, at least they have a chance with us. I don't want to take away that chance."