According to a report in a newspaper the other day, a library in Leeds has taken to labelling its novels with a system similar to that used on ready-meal curries. "Hot" reads are those that are so exciting you begin to perspire (not simply a literary fantasy incidentally - I have myself been reduced to moist-palmed anxiety by a good thriller), "medium" reads are "fascinating books you'll find hard to put down" and mild books are "stress-busters, like Emily Bronte".

This rather startling example immediately raises the obvious problem of categorisation. You can imagine a neurasthenic reader, looking for something soothing after a bout of panic attacks, and pulling Wuthering Heights from the shelves on the strength of that reassuring description. A mere twenty pages into the book they would come across the famous description of Lockwood's nightmare: "I discerned, obscurely, a child's face looking through the window - terror made me cruel; and, finding it useless to attempt shaking the creature off, I pulled its wrist on to the broken pane, and rubbed it to and fro till the blood ran down and soaked the bedclothes."

I hope that the librarian who instituted the system has considered the possibility of being sued for classificatory malpractice. "Your honour, my client has been unable to open anything but Large Print Barbara Cartland since her completely avoidable exposure to the gothic excesses of Miss Bronte's imagination. We will be seeking punitive damages."

Whether the curry grading system will work or not, it does offer another symptom of the dilemma in which the libraries find themselves, obliged to adopt an increasingly consumerist approach to their users by falling subsidies. Ever since the 1964 Libraries and Museums Act sealed the death of commercial circulating libraries, the received opinion seems to be that the "National Health Service for books" has been on a long, slow decline.

This sense of falling off is shared even by the most passionate book- lovers. The writer Ian Breakwell captures the mood: "A people's university in times gone by, a communal seat of learning, reduced to a moribund amenity, like a doctor's surgery without the patients. Dull and dead as a doornail."

If that's what your friends think of you, you really must be in trouble. But the reality is nowhere near as gloomy. According to the Library Statistics Unit at Loughborough University, borrowing has dropped over the last quarter century, from 11.7 books per head in 1971 to 9.2 books per head in 1994, but more than 30 million people are still registered in public libraries, which hardly suggests terminal decline. After years of purely theoretical support for the system, I recently upped that statistic by one, joining my local library. I found a world very different from the one I relished when I was at boarding school.

Almost the first thing you see is a rack of videos, and though this was the very reason I had rejoined (I needed to be able to get hold of classic films) I still felt a shock at seeing the enemy invited through the gates in this way (even if the videos provide a valuable "profit stream" for beleaguered services).

When I take out a video I feel a mild charge of shame, as if I were somehow collaborating in the suffocation of an ideal of literacy. This sensation is not too extreme if the film in question is a silent masterpiece, such as Battleship Potemkin or Intolerance; they feel enough like self-improvement to pass as honorary books. But anything lighter has to be bolstered with an incontestably edifying work, a volume on Kant or a history of Victorian London, rather as a man will buy The Economist to accompany his copy of Hot Babes in Rubber.

There are computers now too, instead of the old wooden cabinets of classification, those long morgue drawers with their rasping brass-tongued handles and tilted decks of index-cards, some still sharp-edged and pristine, others fingered to a grubby felt. Finding a book no longer involves wading with your fingers through the surreal miscellany of titles, any one of which might lead your interest astray and suck you in. Now a few taps on a keyboard take you straight to your destination.

But some values persist. Though the beige cardboard pockets into which your precious library cards were tucked have given way to the bleep of bar-code readers, loans are still finalised by a rattling thump from the date stamp. And, for all the commercialism, there is still the tangible sense that this is a place apart from the world outside - a space in which very disparate groups, of class and race and age, are bound together by a common purpose.

If that sounds hopelessly idealistic and old-fashioned, it's perhaps because the library remains a location where market forces lose some of their power. It feels slightly strange now to take part in a transaction where goods are handed over for free, and where communal trust is part of the deal. They give you something valuable in the reasonably confident expectation that you will bring it back again and there is something a little thrilling about that, if you've lost the habit. It's as if 18 years of Thatcherite bibliophobia (she clearly thought that reading was a kind of idling) has restored the childish sense of wonder you felt the first time you were allowed to take the books home.

What's more, a library liberates you from the tyranny of the recent, that oppressive sense of novelty which hangs over every bookshop and newspaper review page. Libraries are much more generous and broad-minded in their notion of what deserves attention or shelf-space. It is very precious and it can't be taken for granted any more. When we see silly gestures coming from those who struggle to keep the ideal alive, we should remember that they may not be waving but drowning.