Monday's Book: The She-Pope: A quest for the truth behind the mystery of Pope Joan by Peter Stanford (Heinemann, pounds 16.99)

In 1806, after a Hogmanay celebration at the River Red post in the wilds of North America, a Hudson's Bay Company officer scribbled a startling entry in his journal. That evening, an Orkney lad, one of the fur-trading crew, had fallen ill, lay down and "proceeded to give birth to a son".

The name of the "lad" was Isabelle Gunn, and the story of her adventure became one among hundreds of popular 19th-century ballads and stories about female cross-dressers. Next to Isabelle's stunning revelation, the prospect of a ninth-century female Pope, who disguised herself as "John" and was stoned after giving birth during a papal procession doesn't seem quite so far-fetched.

This is just the starting point for Catholic historian Peter Stanford's delightful ecclesiastical detective story. Spurred on by a fleeting mention in a guide to Rome, Stanford plunges into the Vatican archives and goes on to breathe life into John/Joan.

Although the distant period mitigates against any absolute certainty about Joan's existence, she was probably born to English parents in Germany around 855. An extremely intelligent child, she studied at the monastery library in Fulda. There Joan would have had access to stories about other cross-dressing saints, such as Saint Perpetua, a third-century martyr in Carthage, who dreamt while in prison that she was changed into a man to meet the lion's den.

Such a tradition of gender-bending surely inspired Joan to continue an ecclesiastical career, to become John. "Watching her female contemporaries and their elder sisters embark on a life of repeated childbirth and, very often, death by the age of 20 could also have helped her to come to a decision," writes Stanford.

There are passing references to Pope John VIII, aka Joan, throughout medieval manuscripts where "he" is praised for "his" various intellectual endeavours. By the time of the reformation, however, Catholic scholars were busy kicking over traces of John/Joan and soothing gender confusions by dismissing the she-pope as an anti-papal conspiracy.

The church's message to women was unequivocal: "However fine your brains, your sex and your bodies will be your undoing."

An utter delight, The She-Pope continues a tradition of shining light into obscure but fascinating corners of history. If Stanford's careful and wonderfully readable research fails to convince you of Joan's existence, perhaps a visit to the Vatican's Gabinetto delle Maschere will. There sits the now-defunct sedia stercoraria - the pierced chair - where newly- installed popes were required to expose their genitals for a discreet feel, to convince the authorities that they were indeed of the correct sex.