Why not be a writer? If the question sounds familiar, it's because it probably is. An organisation called The Writers Bureau has been posing it in national newspaper adverts for years. You know the ones, they begin with the claim that “as a writer, you can earn very good money”, continue with the offer of a full refund of fees if you don't get published and conclude with a picture of “Christina Jones from Oxfordshire”, an apparently satisfied customer, gurning above the quotation: “So far, I have had 16 novels published!”

The adverts fascinate, not least because no one I know has ever come across any of Christina Jones's books. It's truly remarkable that someone could have written so many words and yet been read so little. But then, as the one-time author of The Financial Times's daily bond column, I know the feeling.

Then there's the intriguing longevity of The Writers Bureau. Countries succumb to dictatorships and are liberated again, Federal Reserve chairmen come and go, Daniel Day-Lewis even gets out of the house to make a new film, but The Writers Bureau plugs on, steadily, throughout.

I understand, of course, that giving up law/medicine/banking to write books is a common fantasy, fed by publicity about the likes of JK Rowling earning millions from book and film deals, Ken Follett being paid trillions for trilogies and Jordan being paid gazillions for regaling us with tales of her moronic life. But, at the same time, isn't the answer to “Why Not Be a Writer?” off-putting? It seems to me, from the business point of view, that there are countless reasons people shouldn't become professional writers, chief among are the facts that:

Most author advances are small. Newspapers like running stories about mammoth book deals, but the numbers are often exaggerations, designed to make agents look like superheroes and debut authors newsworthy and, besides, such authors are in the minority. The brutal reality is that most first-time novelists rarely get more than £12,000 for a two-book deal. Accounts vary, but it is said that JK Rowling got an advance in the region of £2,000 to £10,000 for her first Harry Potter title. Moreover, according to the Society of Authors, the average author earns less than £7,000 a year.

Even large advances don't go far. Say you hit the jackpot and get a £100,000 deal, it's still unlikely you'll be putting in an order for an Aston Martin V8 Vantage. Typically, the sum is spread over a two-book deal, and given in stages — a chunk when you sign, another portion as you hand in a manuscript, another when a book is published and so on. This could mean you get the money over several years and £25,000 a year isn't really comparable to winning the lottery. Especially when 15 per cent will typically go to your agent — and you'll be paying tax as well. This is one of the reasons why even very successful authors have other jobs: Philip Larkin was a librarian; Mohsin Hamid, whose brilliant novel, The Reluctant Fundamentalist, was shortlisted for the Man Booker Prize, is head of consulting at Wolff Olins; and many others are journalists.

It's getting increasingly difficult to earn out an advance. It's called “an advance” because it's a pre-payment of royalties you will earn when the book is sold. But such is the extent of discounting now - supermarkets can demand up to 65 per cent off the cover price - that it's getting harder to earn anything above that sum. If a £20 hardback sells for £8, the author's royalties will also reduce substantially. Meanwhile, the spectre of online piracy lurks ominously over the industry. Electronic book publishing has yet to take off, but writers worry that efforts by publishers to go digital - there was news this week that HarperCollins and Random House are making tentative moves to make new books available for download - will decimate the market in the way that the music industry has been floored by online piracy.

There's no way of guaranteeing a hit. More than any other product, the success of a book is not related to the amount of marketing money thrown at it. Word of mouth is what matters. According to a recent survey, a quarter of readers said that the last book they read was based on personal recommendation, while around a third of under-35s cited recommendation as the most important factor in their latest choice of book. An agent I spoke to recently put it this way: book publishing isn't a business, it's a casino.

Most books disappear without a trace. Last month The Times published statistics from Nielsen Bookscan, which tracks book sales nationwide, showing that, of 200,000 books on sale last year, 190,000 titles sold fewer than 3,500 copies. More devastating still, of 85,933 new books, as many as 58,325 sold an average of just 18 copies. And things aren't much better over the pond: I read recently that, of the 1.2million titles sold in the United States in 2004, only 2 per cent sold more than 5,000 copies.

But despite this, and all the above, and the fact that writing is lonely, fattening, difficult and depressing, because there's always someone doing it better, The Writers Bureau continues to flourish, seemingly, and people keep on wanting to write.

Why? Well, I think it is, in part, because the authors who do succeed, succeed spectacularly: bestsellers sell in bigger quantities than ever. But perhaps more importantly, the desire to write has nothing to do with business. People put pen to paper for deeper, more profound reasons, and I'll expound upon some of these reasons, in a roundabout way, next week, when, as it happens, this newspaper serialises my first book.

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