Frances Wilson, a judge for this year's Whitbread, argues that the really scandalous thing about lit

Literature, according to F R Leavis, should make you a better person. These days we find this querulous categorising of books into those which are good for you, and those which are not, more quaint than instructive. Still, when it comes to judging prizes, we are all Leavisites: a good book is an improving experience.

This, at least, is the conclusion I came to last summer when, as a judge on the biography panel of the Whitbread Book Awards, I attempted to weigh up the relative merits of the 60 or so biographies, autobiographies, diaries, memoirs and confessions that were stacked up around the house like the seven pillars of wisdom. How can you begin to evaluate books in a category as catholic as this? The Whitbread prize, the overall winner of which is announced on 24 January, is awarded to the book that has given the judges most "pleasure", but even pleasures have categories of their own. The pleasure of a lock-in at your local is different from the pleasure of a bracing walk with fine views, and it is pleasure of the sort that makes you a better person that the Whitbread is looking for.

The read that gave me the most pleasure was The Insider, Piers Morgan's diary of his decade as a tabloid editor. It is a juicy evocation of the moral vacuum of the media world, and the pleasure it provides is, need I say, very much of the "wrong" sort. The Insider is a scandalous book whose subject is scandal, but it would have created more of a scandal had it been shortlisted alongside works of outstanding scholarship such as Hilary Spurling's Matisse or of raw emotional honesty, such as Richard Mabey's Nature Cure.

A book can be a good read even when, as Mae West put it, goodness has nothing to do with it. So what value do we give to the pleasure of the scandalous book, in which the author confesses his sins without seeking salvation? Other books that gave me the wrong sort of pleasure were Diana Melly's memoir of her adulterous years with George, Take a Girl Like Me, and Tracey Emin's odyssey of sex and self, Strangeland. Because neither woman learned any lesson or reformed her wanton ways, neither will ever be garlanded with laurels.

Have literary judges always been such moral arbiters? Imagine judging a prize for biography in the 1820s, and having to choose between William Hazlitt's frenzied account of his sexual obsession with a servant, Liber Amoris (1823), Thomas De Quincey's self-explanatory Confessions of an English Opium-Eater (1822), and the stylish, racy Memoirs of the courtesan Harriette Wilson (1825), which was written as an exercise in blackmail on a grand scale and implicated practically the whole establishment, from the king, four prime ministers, the cabinet and shadow cabinet to most titled men in Mayfair and a good deal of the army. Of the three books, all scandalous in their own way, the one by Harriette Wilson (about whom I wrote a biography, which also did not win a prize) comes closest to the problem represented by Morgan's The Insider, because hers is the book that was seen as having the least literary value, in the sense of being least good for you.

Harriette Wilson's Memoirs did for the Regency period what Piers Morgan's diaries did for the 1990s: they caught the era with its trousers down and sold by the sackful. The reason that the quality of both books has so often been overlooked is not that they expose ugly things about the societies in which they live - no one doubts that tabloid editors and society courtesans know more than most about power and corruption. The real causes of their lack of acclaim are less obvious.

For one thing, they both present themselves as an unapologetic part of the system they describe; neither writer asks our forgiveness or suggests how the world could be better. (On each occasion that Morgan admitted to a flicker of guilt at the destruction caused to individual lives by his need to sell papers, he moved an inch closer to my longlist. There were only two occasions.) Second, both authors edit the truth in order to tell a good story. Morgan is said to have dabbled in his diaries at a later date so as to make himself seem like Mystic Meg (he could always, for example, sense that Paula Yates was heading for trouble). Somewhat more dramatically, Wilson scored through names and events as and when she was paid to do so, and inflated the status of various characters according to their treatment of her. So while Morgan's diaries read more like a thriller than the residues of the day mulled over at bedtime, Wilson gives the Duke of Wellington (who responded to her attempt at blackmail with "publish and be damned") a more significant role at her own bedtime than he probably had. Both books, as such, are scandalous not because of their embarrassing and exposing authenticity, but because they are not authentic enough.

Literary prizes and scandal go together like magnets and fridge doors, but the real scandal of the literary prize is that it is rewarded for virtue. I would have been no more able to reward Harriette Wilson in 1825 for the pleasure given by her writing than I could Piers Morgan in 2005 for the pleasure afforded by his. As a culture, we award merit to narratives that are both factually accurate and redemptive, areas in which both Harriette Wilson and Piers Morgan quite magnificently fail.

Frances Wilson is an author, biographer and critic, whose works include The Ballad of Dorothy Wordsworth. Her most recent book is How to Survive the Titanic, or the Sinking of J Bruce Ismay. She reviews for the TLS, the Telegraph and the New Statesman.

This article first appeared in the 23 January 2006 issue of the New Statesman, Why British men are rapists