We are living in a golden age for biography, declares Nigel Hamilton at the outset of his primer on how to write one. He must be living in a universe parallel to the one that I inhabit. In Britain, the heartland of a great biographical tradition, the popularity of serious biography has been plummeting for some time now. Long gone are the days when a book of the stature of Richard Ellmann's Oscar Wilde could sell 75,000 copies in the UK alone. In the United States, where Hamilton lives, biography has never had much of a fan base (witness bookshops in the United States, where lives of present-day politicians and film stars are the only biographies allowed to bulk large).

As sales have continued to slide, the poor biographer has borne the brunt of the inevitable consequences, finding himself at the mercy of penny-pinching publishers who are increasingly unwilling to furnish him with the advance he needs for his four years of research and writing. Historical biography fares better in this climate than its literary counterpart, which, with a few notable exceptions, is all but dead in the water, though even here the only safe investment is in big names - a combination of famous subject and well-known author. Meanwhile, biography is going through an identity crisis, experiencing one of its periodic fits of anxiety. Should it ally its destiny more closely with fiction, or stick to its conservative appeal as a purveyor of authenticated facts (Virginia Woolf's old "granite and rainbow" routine)? One thing is for certain - in the continuing shifts of taste among the reading public, publishers are generally the last people to spot a new fashion or prevailing trend.

Despite this discouraging scenario, Nigel Hamilton is determined to explore the tools of the craft to benefit a new generation of would-be biographers waiting in the wings. His credentials are impressive. He is the biographer of Field Marshal Montgomery ("the longest biography ever undertaken of a British World War II commander"), of JFK, and of a projected three-volume life of Bill Clinton, two volumes of which have appeared so far, causing consternation among some critics because of Hamilton's seemingly justifiable concentration on the president's sexual peccadilloes. In addition, Hamilton is the author of Biography: a Brief History, published last year, where he recorded his failure, together with that of his fellow trustees, to set up a national centre in London for the study and celebration of biography - the British Institute of Biography, known affectionately as BiB.

All this is very worthy and commendable, but Hamilton's new guide, though undoubtedly purposeful, is also rather ponderous. There are chapters on choosing the right subject - make two lists, he says, before you decide, one of positive, the other of negative qualities - of defining a book's readership, on writing about childhood, love affairs and the twilight years. Some of Hamilton's advice seems blindingly obvious: "Contact relevant libraries and archives which may have material pertinent to your subject", or make sure you're equipped with a "sturdy and reliable tape recorder" when interviewing witnesses. Once, I must confess, he made me laugh out loud when he suggested that humility was one of the prerequisites of a biographer. You won't find much evidence of this characteristic among the ranks of the self-appointed leaders of the profession in Britain today.

At several points, Hamilton bemoans the absence from the American university curriculum of any courses on biography. In Britain, by contrast, life writing, to use the modish description that encompasses autobiography as well as biography, is becoming well established in university departments, though it is unclear to what end exactly. After decades of treating biography with the type of contempt usually reserved for uncultured cousins, the academy has embraced the genre with an alarming zeal. Teaching life writing is often a useful way for practitioners of the craft to supplement their income, but such courses may turn out in time to be the scourge of the modern age, as the market is flooded with the personal memoirs that students have been encouraged to write as exercises in self-expression. "You may decide you wish to write about yourself, rather than somebody else," notes Hamilton. One suspects that this may indeed become the rule rather than the exception.

What finally is missing from Hamilton's primer is any sense of the imaginative empathy of a biographer for his subject that always used to be the bedrock of great biography writing. It is this kind of inventive spark that will at some point rejuvenate the genre in the way it always has done in the past. In what form this will occur, it is impossible to predict. Given that our ideas of what constitutes biographical truth are less clearly defined than they once were, the solution is likely to be neither obvious nor straightforward.

Mark Bostridge's "Florence Nightingale: the Woman and her Legend" will be published this autumn by Viking