I don’t know when I started writing in my books, but I know when I started to notice. I was in my mid-20s, recently graduated and relearning the pleasures of reading for myself, which was part of learning how to be in my own company. I’d moved to a closet-sized studio apartment crammed with stuff I loved, and I was building my nest in the world with the standard invigorating arrogance. Writing in books felt like coating the text in my enzymes, assimilating it into whatever I thought I was becoming.

It was around this time I read Marginalia by H.J. Jackson, the first formal, book-length study of the practice. Jackson is a former professor of mine, and a scholar of Samuel Taylor Coleridge, who was known for his voluminous margin notes — friends would often ask him to mark up their books as a favor — as well as for popularizing the term “marginalia.” Jackson’s book, for which she examined over 2,000 manuscripts, is a general history of marginalia as a social and personal practice, as well as a study of its uses and motivations. Annotation is, Jackson writes, a process of identity formation: “A marked or annotated book traces the development of the reader’s self-definition in and by relation to the text. Perhaps all readers experience this process; annotators keep a log.”

Forming identities in public is one thing, shedding them is another

“Writing in books is the closest I come to regular meditation,” the critic Sam Anderson wrote in a New York Times Magazine essay I remember clipping when it first appeared. (Anderson has published his own marginalia, in the Times Magazine and the Millions.) Marking up white space, he said, is “a way to not just passively read but to fully enter a text, to collaborate with it, to mingle with the author on some kind of primary textual plane.” It was 2011, a year after the iPad was introduced; e-readers were still relatively novel, and Kindle had just rolled out a new feature that would allow readers to mark up their e-books and share the results — “Coleridgean fantasy software,” Anderson called it.

Any new format requires you to examine familiar customs, and many writers at the time were set on defending their attachment to paper books: partly haptic, partly aesthetic, partly the resistance that comes from having invested a lot of money in objects that risk becoming worthless (this is why I carried around a Discman, along with about 10 pounds of CDs, well into the decade). But Anderson was excited about digital possibilities for annotation. “Marginalia — with its thrill of shared immersion — is what the culture is moving toward, not away from,” he wrote. “We are living increasingly in a culture of response. Twitter is basically electronic marginalia on everything in the world.”

Back then, Twitter was still sometimes referred to as “micro-blogging,” blogs themselves having descended from private journals. In the years since, marginalia has come to seem like a much more appropriate analogue, and Twitter something like an evolutionary step for annotation itself: a dedicated venue for readers responding to text and to each other’s responses, so intricately that the text becomes secondary, valued primarily for what can be said about it. The interface between reader and text — that “primary textual plane” — is more tangible and, of course, public.

Margin notes are a little like journals, but somehow both more social and more compromising. Journals, I’d argue, are relatively considered, declarative, and generally addressed to a reader with more knowledge and better sense than you had at the time of writing — either yourself at a later age, or a future human presumed to be interested in the minutiae of your life. Margin notes, on the other hand, are one side of a dialogue, which any reader would only be listening in on. When I think of a stranger reading my margin notes, it calls to mind the feeling of sheepishness I get on a quiet streetcar after a friend with whom I’ve been chatting has reached their stop.

Twitter was once a smaller, more insular space, and tweeting in 2011 felt more like scribbling unselfconsciously in a book. I was discovering things about the world and myself, excited by what I found, and it was affirming to be able to publish something and discover that three or four people found interesting what I found interesting, or funny what I found funny, or galling what I found galling. My closest friends were in the same stage of life, and social media were venues for our hope and excitement — which seems viscerally out of pitch with the people we are now, the worlds we’re engaged in, our horizons of concerned. Forming identities in public is one thing, shedding them is another.

Those anxieties had to do with the sense that with every post, every click, I’m shedding a doppelgänger

Half a decade later, already a different era, I packed up my tiny apartment in the process of moving to a bigger city. I piled up all the books I had no intentions of rereading and thought with some initial excitement about who I might pass them along to — who might pick them up from the curb — and how their impressions might align or collide, at least in the abstract, with mine. Then I opened one, and saw my own strident observations shrieking up at me in bleeding ink. The notes seemed foul, the waste products of a self I’d repudiated; there are few people more objectionable than the person you were until recently. As objects, the books seemed cursed in reverse: To most readers the notes would be nothing more than an eyesore, but to put them in circulation would somehow manifest versions of myself that no longer felt familiar, and seemed to risk preceding me.

The fear wasn’t rational, of course — it was an incarnation of certain anxieties I’d developed over years of living online, transposed to objects. Those anxieties had to do with the sense that with every post, every click, I’m shedding a doppelgänger. We have so little control over the fate of our communications, or the way we’re profiled by third parties who assemble versions of us out of decisions we barely remember having made. These doppelgängers are strangers, with lives of their own over which we have no control. It feels uncanny in the mode of decomposition, the question of what becomes of the body after you’ve ceased to occupy it.

When I was 25 and cocky, I thought my margin notes might have some inherent value, at least to the future me — that my thoughts and opinions were valuable, that I needed to set out pots and pans to collect them all. Rereading them now is, when not mortifying, dull. From a distance, though, this general attitude seems like a healthy delusion for a young person trying to slough off the sense of inadequacy and assert themselves in the world. If the notes have any value, it’s impersonal — they’re in some way documents of a joyful, vivid stage of life, where one is excited not only by ideas but the idea of ideas, and discovering the kind of life they’d like to lead.

What I loved about Jackson’s book is how it spoke to a humane theory of reading, one that focused on the relationships we forge with our favorite (and least favorite) texts, as well as other readers, over the sanctity of great literary genius — how it seemed to consecrate marginalia as both phenomenon and metaphor. “Somebody introducing me once said the book was about the marginalia of famous people, whereas in my view it was exactly the opposite,” Jackson told me when I interviewed her a few years ago. “It was really a way of exploring the marginalia of anonymous people, or complete unknowns, people with no names at all and of no importance.”

Marginalia felt to me like a hybrid of scholarship and self-help: in the end it recommends marginalia as a shared personal practice, one that feels even more useful against the pressures of living in public. Books might incarnate that doppelgänger fear, but they soothe it, too — a book can be opened and closed, and kept on hand.