Howard Jacobson: What are libraries for? Tramps, filth and erudition - not soul-destroying detritus

Several years ago, when the British Library was housed in the British Museum - which meant that you could always while away the odd hour looking at ancient Greek or Indian erotica if you didn't feel like work - I had cause to ask to be delivered to my desk The One Hundred and Twenty Days of Sodom by the Marquis de Sade.

Today it's freely available in print, for all I know at a reduced price in Tesco; but 20 years ago you had to sign for it, your signature being an assurance that you required the book for purposes of serious scholarship only, that you would not deface or remove it from the library, that you would not divulge to anyone what it contained, and that you would think clean thoughts.

I've invented some of those stipulations, but not all. Before the British Library would entrust a volume of such gross indecency to your safekeeping, it did indeed insist you complete a form promising not to reveal a word of what you'd read. Since I was researching De Sade for a novel about someone who was researching De Sade, I didn't see how I could honour this undertaking, and for months after my novel was published I would start whenever the doorbell rang, fearing that the British Library Police had come for me.

What they did to people who reneged on their assurances and put the contents of restricted literature into the public domain I had no idea, but given that this was not any old restricted literature, but The One Hundred and Twenty Days of Sodom, I didn't see how the punishment could fail to measure up to the chastisements adumbrated in that work. Embuggery, amputation, dismembering, slitting, slicing, cleaving, the rack, the cross, the guillotine, flogging with a bull's pizzle while being forced to kiss a nun's... But there I go, blabbing again.

What has brought these hours of innocent study in the British Museum back to me after all this time is the suspension last week, pending investigation, of Ceri Randall, librarian at Pyle Library, near Bridgend in south Wales. Under investigation is whether Ceri Randall was within her rights to blow the whistle on a person she believed she had seen demonstrably doing porn on one of the library computers. What constitutes fit material for a public library - that is the question.

Just about nothing that's in it these days is my first response. Call me a pedant, but I think of a library as a place that houses books. Books which educated opinion deems us to be the better, intellectually and spiritually, for having read. If you wonder who should be given the responsibility of deciding which those books are, wonder no more. I will do it. So call me paternalistic as well.

It amazes me that we have to go on insisting on this. The idea of a free library presupposes the value, to the individual and to society, of reading, and the value of reading presupposes the value of books. If we fill a library with potboilers and that genre of contemporary literature described as crossover because it crosses us over from maturity to infancy, we abandon the grand educative function which libraries were philanthropically invented to serve. First, the serious books give way to footling books, then the books give way altogether to something else. Records, tapes, CDs, DVDs, and now computers.

Don't mistake me for a puritan. I like the lunacy of libraries. I like the tramps pretending to be immersed in newspapers, and the people who have been swindled of their inheritances trying to put together lawsuits from the only law book on the shelves, and the would-be aristocrats searching family trees, and the general-knowledge freaks memorising every entry in the Encyclopaedia Britannica, and the mutterers and the snorers and the wild laughers and the rheumy old men who are here every day, from nine in the morning to six at night, shouting "Shush!" at anyone who coughs. Libraries attract nutters - it's the flipside of their grand educative function - and it's proper that whoever haunts books should be kept in mind of the fragility of reason.

Books sometimes make you wise, and sometimes send you mad. But the detritus of popular entertainment, which leads neither to wisdom nor to madness, only to terminal triviality, and from which any good library should be a refuge, is something else again. Don't give libraries a penny, I say, until they present themselves once more as palaces of bookish learning, for the behoof of the studious and the deranged alike.

So Ceri Randall is a bit of a hero to me, by simple virtue of the fact that she has been prepared to make a value judgement in a library. What you are doing here is low and inappropriate. It was not for swapping indecencies in an internet chat room that libraries were built. Get out!

All I hope now is that she keeps her job and starts evicting any adult she catches reading Harry Potter.

But then I remember my library experiences with the Marquis de Sade, and the look of utter distaste which crossed the librarian's face when I asked whether the ban on telling anyone what I was reading extended to my wife. My wife? I would speak of embuggery to my wife? For a moment, she made me feel I had befouled a sanctum - no, two sanctums, the sanctum of marriage and the sanctum of scholarship.

There is a difference, of course, between De Sade and a chat room. One is literature - oh yes it is, forcing us to re-examine "the true relation between man and man", in the words of Simone de Beauvoir - and the other is the mere vacant rubbing at an itch. I would like Ceri Randall, as she prepares her defence, to be very clear about this. Filth isn't the issue. Filth can be art and art belongs in libraries. The issue is the trivialising of the human soul. And libraries aren't for that.