I am almost neurotically law-abiding, but there is one area of life where I am an outlaw, beyond the pale, a fugitive from justice. I only do it in pencil, and sometimes I remember to rub it out, but … I write in library books. Those spaces down the sides of the page seem so inviting that the impulse to anoint them with scribbles is irresistible. History is on my side: until the 19th century books were often used as scrap paper, and few people had qualms about scrawling on a pristine copy. No jury in the land would convict me. Books are meant to be written on.

Is such annotation a dying art in our online era? Most ebook readers allow you to highlight text and take notes, but there isn't the same aesthetic of columns of alluring white space. On the other hand the web has whetted our appetite for sharing reading experiences. Amazon has just introduced a facility for the Kindle which posts your marginalia online so others can read it. Social reading websites like BookGlutton, where you can attach notes for other readers of the same book, have been around for a while.

You could argue that this impulse is really a return to the great age of marginalia, which the literary scholar HJ Jackson identifies as lasting from about 1750 to 1820. The practice then was widespread and communal. Samuel Taylor Coleridge, who coined the word "marginalia", wrote his own marginal comments with an audience in mind – and even published some of them. "You will not mind my having spoiled a book in order to leave a Relic," he wrote, a little smugly, in one of Charles Lamb's books. Many of today's social networking sites similarly create a kind of ongoing collective commentary – not just on books, but on the world in general.

And yet there is something missing from this electronic marginalia. First, it seems so ephemeral. Pencil marks left on a page will last several lifetimes, perhaps as long as the paper itself. Public Notes, on Kindle, are less tangible and, even if someone is archiving them, are likely to be unreadable in future because of hardware or software changes. The most basic motive for writing marginalia is surely to create a sense of ownership: children often write their names over and over again in books. You can't do that with a Kindle.

Second, this public note-taking seems too much like performance. For the last two centuries, marginalia has been semi-private, almost furtive, a silent communion with the author or the unknown reader who might pick up the book, secondhand, a generation later. Marginalia is, by definition, something on the margins – undervalued, overlooked.

Such textual detritus is a voguish topic in academic literary studies. The reading experience database at the Open University, for instance, aims to map all evidence of recorded engagement with texts from 1450 to 1945: everything, from marginalia to commonplace books, that might show that ordinary people actually read books rather than using them to furnish a room. This kind of evidence is surprisingly sparse and often resoundingly banal. Many readers just wrote dictionary definitions in the margin, or "v good", or they simply underlined words. And yet this sub-Coleridgian commentary can also be rather touching, evidence of an earnest wrestling with meaning and thirst for knowledge. True marginalia is an end in itself, a brave attempt to bridge the ultimately impassable gulf between writer and reader.

And because marginalia is unguarded, meant only as an aide-memoire, it can accidentally reveal much about the person writing it. Releases from the national archives disclose that Margaret Thatcher wrote words like "No!" and "This will not do" in the margins of draft documents. I suppose it would have been more newsworthy she had sprinkled the margins with encouraging words and smiley faces, but you can't deny that her marginalia sums her up perfectly.