Access to the Cambridge college libraries is generally limited to members - undergraduates, postgraduates and Fellows - so the security of collections has never been a particularly grave concern. This is not to say that college librarians don't worry about the illicit removal of books. The secret all librarians jealously guard - that a small part of every open collection will inevitably go missing, never to return - is as true of Cambridge as anywhere. We employ various technical and administrative measures to prevent book theft, but until a recent move from one college to another, I had never considered the book curse.

I read it on my first day, printed dot-matrix style on a piece of card. It was left next to a self-issue computer as a warning to those who might think of attempting to bypass the technology.

"For him that Stealeth a Book from this Library/Let it change into a Serpent in his hand & rend him/Let him be struck with Palsy, & all his Members blasted."

It continued, eight lines in total, all Ye Olde Englishe physical horror. The card claimed it was a 16th-century Spanish book curse from the monastery of San Pedro, Barcelona.

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The curse was too good to be true and immediately rang linguistic alarm bells. Iberian monks from the 1500s were not noted for their fluency in Early Modern English. If real, it must therefore be a translation. But from a monastery in the Pa?sos Catalans at the end of the Valencian Golden Age, the curse is unlikely to have been written in Spanish. Any modern translator of the original Catalan (or, more likely, Latin), would be unlikely to daub it with archaic embellishment.

It cannot have been translated in the 16th century either - it is historically unlikely that there was enough communication between Britain and Barcelona in the 1500s for the borrowing. The early 18th century brought British soldiers into the region, but by then the English third person singular had lost its -eth inflection. Finally, the curse speaks of bookworms, which, as a word to describe book-eating maggots, was not used until the mid-19th century.

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The book curse from the San Pedro monastery is no such thing. It is an amusing hoax dating from 1909. Edmund Pearson, joker, librarian and true-crime writer, claimed it was part of a rediscovered Old Librarian's Almanack originally published in 1773. The Old Librarian dated the translation to 1712, "English'd by Sir Matthew Mahan" in a guide to his Spanish travels.

Pearson's imposture was taken as a genuine discovery on its publication and received favourable reviews, although the forgery, never completely hidden, was quickly uncovered. However, the memories of librarians are short, and few of the library websites that repeat the curse (Google finds it over 5,000 times, and in some very respectable places) question its authenticity. The internet has allowed it to prosper. But to those who might consider the illicit removal of library books, the fact that it is inauthentic is irrelevant. In calling attention to the evils of book theft it might be said to be doing its job, no matter who wrote it.

Myths grow in libraries like pearls in oysters. Librarians sometimes start them, only for them to race away with lives of their own. One pities the early 20th-century British Museum attendants harassed by left-wing visitors wanting to know exactly where Marx had written Capital. In truth there was no such desk - Marx sat where chance took him, and he did his writing at home in the evenings, after the library had closed. Yet someone, annoyed perhaps by his fifth Communist day-tripper that morning, must have blurted out "O7", and from there the myth escaped.

Marx's presence in that iconic space is itself a self-propelling myth. Although he did spend considerable time in the museum's Round Reading Room during the late 1850s, he was never the daily attendee described in countless badly researched biographies. He did not have a reader's pass to the British Museum for the first year of his London exile, nor for much of his last 15. The Round Reading Room did not actually open until seven years after the start of his English exile. Yet the myth of Marx's quotidian presence, first alluded to in populist fellow-traveller accounts printed in the first half of the 20th century, and burnished by continual journalistic references in the second, has become received wisdom.

The bigger the library, the more audacious the myth. The genesis is clearly the Library at Alexandria, nearly everything about which is hypothesis, supposition, speculation and projection. It almost certainly did not exist in the form envisaged by popular, or even academic, culture (an amusing pictorial representation of which can be found in its Wikipedia entry). Contemporary libraries are likewise magnets for fanciful solecisms. I have heard it said, and seen it written, that the Bodleian Library has miles of underground tunnels leading to bookstacks under much of central Oxford. Instead, there is only one tunnel, soon to disappear in its current form, and I am lucky enough to have been through it.

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Since its erection in the early 1930s, the tower of the Bodleian's great rival, Cambridge University Library, has been spoken of by generations of students as being a storehouse of pornography. The truth is more prosaic - the books are mostly Victorian, not exactly a time of sexual licentiousness. The reason why the tower is crammed full of smut, I once overheard a student explain to a credulous tour group, is that "it contains, like, every book ever printed".

People believe these things of libraries. The most myth-soaked novel of recent years, Dan Brown's The Da Vinci Code, contains this immortal line: "I've got to get to a library...fast!" The most explicit demonstration of the link between libraries and myth in popular culture however, must be the (mildly ironic) trilogy of The Librarian films, with Noah Wyle as a newly minted librarian in the mould of Indiana Jones.

In his first outing, the eponymous librarian travels first to the Amazon, then to Shangri-La, to discover "the spear of destiny", ownership of which will bestow the power, inevitably, to rule the world. The second film sees Wyle travelling to King Solomon's Mines, where he finds (and destroys, later confessing that it was "not very librarian" of him) a book full of incantations for controlling space and time.

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Ratcheting up the absurdities even further, the third adventure involves Judas Iscariot, Count Dracula, the philosopher's stone, the KGB and voodoo. The "Metropolitan Public Library", where Wyle's character works, appears to contain not one myth, but all of them, housing not only books, but also the Fountain of Youth, Pandora's box, the Golden Fleece, the Holy Grail and Excalibur, among other objects. The films are wonderfully enjoyable, and within the alternative reality they create, internally coherent and credible.

Libraries have more than once been described as secular churches - places of reason, learning and rationality, not superstition, tall tales and nonsense. So why should libraries and myths fit so well together? The answer to this is likely to be as complex as the myths themselves.

For one thing, many of the great libraries are old, and full of old things. Until recently, even newly built libraries favoured elements of Classical architecture, and it is easier to imagine fanciful things happening in the past than in the present. The Librarian films have contemporary settings, but there is something deeply nostalgic about them.

Many smaller libraries built in the late 19th century were in the Gothic style, leading, inevitably, to alleged hauntings. It has been noted by a number of writers that the emergence of public and semi-public libraries led to a revival of interest in Classical ideas on both sides of the Atlantic. (Marx may not have used the British Museum very much, but Friedrich Engels was a frequent visitor to the Athenaeum in Manchester). In the popular imagination, Neo-Classical buildings have easy associations with public participation, democracy, education and the rule of law. But ancient Greece also brings to mind some of the most powerful myths of the Western imagination.

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It is therefore unsurprising that our storehouses of stories, built in the style of an age that believed its myths to be real, might be suitable places about which we can make our own fictions believable.