Last year I joined the library at the University of Texas, Austin, and rediscovered a literary form I hadn't encountered much since my student days: readers' inscriptions in the margins of library books. The conventions of the genre are simple: you state something obvious in a fragmentary/declaratory style, adding a question mark, exclamation mark or ellipsis according to the degree of confidence you have in your perceptions.

The classic example would be my discovery of the astonishing critical insight "Satan is the hero" inscribed alongside one of Lucifer's speeches in a secondhand copy of Paradise Lost. What motivates readers to write such unnecessary, moronic comments in the margins? My guess is that since most students are young and inexperienced, they find it reassuring to physically capture the first 'critical' comment that flops into their heads as a method of coping with the fact that they are totally out of their depth.

Thanks to UT library, I have recently made several exciting discoveries in the genre. Apocalypses by Eugen Weber is an excellent history of 2000-plus years of End of Times thinking, but reading it wouldn't have been the same experience without the sarcastic remarks about dead prophets an earlier reader had scrawled throughout the book. Next to Weber's account of 12th-century monk Joachim of Fiore's idea that history progresses through three stages towards an era of universal felicity, this acerbic critic had written: "Looks like old Joachim was full of beans!" Reading his many other crappy quips, I suspected he had checked Apocalypses out purely for the purpose of displaying his contempt for religion. If his posturing before an unseen audience of future readers had been funny, I would have forgiven him. Instead, it was a tedious display of ego, like being stuck in traffic behind a car covered in bumper stickers broadcasting the driver's views on everything from Iraq to the success of his kids at high school.

I encountered a more interesting example of margin prose in an essay by historian Pauline Moffitt Watts on Christopher Columbus's obsession with his role as harbinger of the Last Days. It's easy to find evidence of this in Columbus's later life: he edited an anthology of apocalyptic texts called The Book of Prophecies, and asked his royal sponsors to support him in a new crusade for the liberation of Jerusalem prior to Christ's imminent return. However, in order to establish that the younger Columbus also longed for the End, Watts turned to a manuscript in his personal library, which he had used while preparing his first voyage. Columbus had filled the sections on the apocalypse with comments and drawings, such as a big hand pointing at the 'seven signs' of Antichrist. Thus, by studying Columbus's notations in the margins of another man's book (and combining it with other evidence), Watts claimed to have revealed the apocalyptic bent of his mind prior to the first voyage in 1492, arguing that he had always hoped his discovery of a passage to the Indies was part of the drama of the Second Coming.

From a personal perspective, however, the most fascinating example of margin prose I have encountered was inside a first edition of Alone Through the Forbidden Land, by Gustav Krist, published in 1938. The book is an account of an Austrian adventurer's clandestine journey through the states of Soviet Central Asia in the mid 1920s. At first, the marginal author was quite restrained. To Krist's claim that the few Europeans who visited Bukhara prior to its absorption into the USSR were usually sold into slavery or executed, he responded with a simple "?"

Soon, however, his comments took a turn for the pedantic. He disputed Krist's claim that the journey from Barfurush in Persia to Meshed i Sar on the Caspian Sea is a "27 or 28 mile ride" with "?Why 28? It is only 8." (Actually, according to the Encyclopaedia Britannica it's 15.)

Later, when he was especially irritated, he resorted to frustrated outbursts: "Nonsense!" Or simply: "!!!" And: "???"

But Krist's book is a colourful, subjective memoir, written years after the events he described. Only a fool would expect total accuracy. This invisible dullard was misreading it so he could brag to an unseen audience: "I know better." And still I was sceptical. Central Asia in the 1920s was closed even to Soviet citizens from outside the region. Very few westerners had visited the places Krist saw before they were absorbed into the USSR, and even fewer would see them over the next seven decades. I assumed the marginal author was critiquing Krist not from experience but from a dry overfamiliarity with other texts – until I read this passage from Krist:

"The Bukhara that I knew had been an independent state under its own Amir. It had been forced to permit Russian soldiers in its frontier towns, but had successfully prevented the entry of any European into the Holy City itself."

To which the phantom reader replied: "! I have lived there as often as I wanted."

Suddenly, I realised that he was not some petty, sniping boor but, rather, another eyewitness. Texas is not exactly awash with English-speaking European refugees from central Asia, and it was even less so before 1991. Nor am I aware of any early-20th-century memoir of Bukhara written by a rancher, oilman or even UT academic. So who was he? Alas, no answer can be found in the annotations he made to Krist's book.

Nevertheless, as I read the rest of the book, this marginal author changed from a nuisance to a haunting presence; a shadow reader peering over my shoulder. I experienced his notes and Krist's text as a discussion between two experts, tinged with bitterness and jealousy on the part of the Texan, whose experience had not been preserved for posterity. So when Krist writes, "The Bukharans had a few pieces of artillery," and the Texan angrily snaps "Two", I am fascinated but also frustrated. I want to know more, but these scattered, anonymous fragments in the margins of another man's book are all that remains of the mysterious Texan of Bukhara.