Locked in a climate-controlled vault at the Newberry Library here, a volume titled "The Pen and the Book" can be studied only under the watch of security cameras.

The book, about making a profit in publishing, scarcely qualifies as a literary masterpiece. It is highly valuable, instead, because a reader has scribbled in the margins of its pages.

The scribbler was Mark Twain, who had penciled, among other observations, a one-way argument with the author, Walter Besant, that "nothing could be stupider" than using advertising to sell books as if they were "essential goods" like "salt" or "tobacco." On another page, Twain made some snide remarks about the big sums being paid to another author of his era, Mary Baker Eddy, the founder of Christian Science.

Like many readers, Twain was engaging in marginalia, writing comments alongside passages and sometimes giving an author a piece of his mind. It is a rich literary pastime, sometimes regarded as a tool of literary archaeology, but it has an uncertain fate in a digitalized world.

"People will always find a way to annotate electronically," said G. Thomas Tanselle, a former vice president of the John Simon Guggenheim Memorial Foundation and an adjunct professor of English at Columbia University. "But there is the question of how it is going to be preserved. And that is a problem now facing collections libraries."

Marginalia was more common in the 1800s. Samuel Taylor Coleridge was a prolific margin writer, as were William Blake and Charles Darwin. In the 20th century it mostly came to be regarded like graffiti: something polite and respectful people did not do.

Paul F. Gehl, a curator at the Newberry, blamed generations of librarians and teachers for "inflicting us with the idea" that writing in books makes them "spoiled or damaged."

But marginalia never vanished. When Nelson Mandela was imprisoned in South Africa in 1977, a copy of Shakespeare was circulated among the inmates. Mandela wrote his name next to the passage from "Julius Caesar" that reads, "Cowards die many times before their deaths."

Studs Terkel, the oral historian, was known to admonish friends who would read his books but leave them free of markings. He told them that reading a book should not be a passive exercise, but rather a raucous conversation.

Books with markings are increasingly seen these days as more valuable, not just for a celebrity connection but also for what they reveal about the community of people associated with a work, according to Heather Jackson, a professor of English at the University of Toronto.

To read the complete article, see:Book Lovers Fear Dim Future for Notes in the Margins(www.nytimes.com/2011/02/21/books/21margin.html)