In the old days of literature, only the very thick-skinned – or the very brilliant – dared enter the arena of literary criticism. To criticise a person's work required equal measures of erudition and wit, and inferior critics were often the butt of satire and ridicule. Nowadays, though, almost everyone wants to be a critic, and thoughts and opinions are freely expressed on every subject imaginable.

The internet makes it easy, of course; but the advantages of easy access and instant response can sometimes be outweighed by the vast numbers of people out there whose opinions are maybe too freely given, without a great deal of knowledge or thought. Amazon's new feature – an automated programme that extrapolates "blurbs" from readers' reviews – has already caused hilarity by highlighting such insightful comments as: "The Picture of Dorian Gray is a novel by Oscar Wilde", "I read this on my holiday to Majorca", and "time travel is lame".

Sadly, nowadays professional critics are sometimes no more informative. And some give away too much. My concern about the conduct of literary critics stems in part from an article that recently appeared about my latest novel, Peaches for Monsieur le Curé, in which almost every plot twist was given away.

Other critics have personal axes to grind, inadvertently revealing more about themselves than about the author they are reviewing. A few are intellectually insecure, believing (in much the way cannibals once believed that eating an enemy's brain would make them smarter) that sneering over a colleague's work will somehow enhance their own achievements. As authors, we all expect criticism from time to time, and we all have our ways of coping with unfriendly reviews. Some are disciplined enough never to read reviews at all – the rest of us mortals, whether we admit it or not, often spend more time than we should obsessing about reactions to our work, uncomfortably balanced between the fear of not being reviewed at all, and that of being savaged for our success.

I rarely agree to write reviews. I learnt my lesson a few years ago, when the editor of a national newspaper, having commissioned an 800-word review of a colleague's novel, cut my piece down to 400 words, removing all the positive comments and adding a bitchy heading, thereby changing my balanced review into a mean-minded hatchet job. Literary criticism is not a series of negatives. Personally, I'd rather review a book I love than give column inches to one I don't; but I try to bear in mind the fact that even if a book fails to appeal to me, that doesn't make it worthless.

Personal feelings aside, it seems that the business of reviewing – be it books, music or films – has lost a lot of its former style, as well as the respect of the general public. Reviewing at its best is an art as subtle as swordplay; it should never descend into self-promotion, or sour grapes, or name-calling. Too often nowadays, personal attacks have taken the place of legitimate comment – for instance, look at the way in which, for certain reviewers, Téa Obreht's Orange-Prize-winning novel The Tiger's Wife became an excuse to denigrate her age, nationality and appearance. This reflects the attitudes already well-established in the film and music industries, where public figures are seen to be "fair game", and where misogyny, intrusion into personal life and cheap abuse disguised as wit seem to have become the norm, with interludes of fawning sycophancy towards a small selection of sacred cows.

Of course, reviewers should be free to express their opinions, negative or otherwise. But, to be respected as a critic, one has to show a certain respect, both to the writer and to the readership. This means approaching the task with an open, educated mind, free of bias or ego. It means doing a professional job – otherwise, we deserve no more respect than those writers who post anonymously on Amazon, giving their own work five-star reviews and trashing the work of their rivals.

No one should be so precious as to refuse criticism of their work. But to respect an opinion, we have to know that it was given honestly and with proper thought. Reviewers who haven't read the book (one of my novels was once described in a national newspaper as "another of Harris's sweeping historical romances" – it was actually a crime novel set in the 20th century) run the risk, first, of making themselves look ridiculous, and, second, of alienating still further the very readers who keep them – and all writers – in business. Similarly, those who deliberately give away key moments in books or films (as just happened to me), thereby spoiling the experience for anyone who reads the piece, are guilty of the worst kind of arrogance, abusing their position to score one over a colleague at the expense of the general public.

At a time when unpaid bloggers online are gaining influence at the expense of professionals, we need to convince the public that good reviewers exist, and are still worth listening to. Otherwise, our readers will continue to look to the internet for news, and the art of the book review will join the typewriter in the trashcan of Time.

Joanne Harris's novels include 'Chocolat' and, published later this month by Transworld, 'Peaches for Monsieur le Curé'