Is it OK to write in a book? Death to those who write in library books, but I say if you own it, scribble in it. Tribune writer Patrick T. Reardon is too "persnickety" to do it, but even a ruthless reader like Tyler Cowen is reluctant to write in books. He explains, "I wish to discover new ideas -- and not just my old ideas -- each time I open them up." Fair enough, although I would ask him how the old ideas come to be without writing them down someplace.
I write in books all the time, although rarely in novels, for some reason. My notes are generally pretty dunder-headed, just bland summaries. For the most part I'm just mapping out the text so I can quickly locate topics later. Here's a sample from a book I just started reading yesterday, Mark Poster's Information Please: Culture and Politics in the Age of Digital Machines: "M.E. [Middle East]--maintain autochthony of Islamic culture." I made this note partly to remember how to spell "autochthony," a word I like, and partly to situate the term "aberrant decoding," which may or may not come in handy later. At the top of the page I've written "Bert is evil," which means something in the context of Poster's text.
Or is it my text? Information Please is Poster's intellectual property, but the book is mine. I wouldn't have bought it if it didn't already contain something of mine. Marginalia is my way of appropriating pieces of texts that are objective correlatives to ideas rattling around in my head. There's a moment in Limited Inc in which Derrida realizes the degree to which someone else's text contains his (i.e., Derrida's) ideas. One day Derrida was reading a prepublication manuscript of John Searle's when Derrida noticed a copyright on a manuscript about Derrida's writings. The copyright reserved for John Searle all rights to a text about Jacques Derrida. Derrida retorted by deconstructing this little tidbit of marginalia to show that nobody owns a discourse, that copyright law ends when reading begins. In fact, Derrida's oeuvre can be read as an exercise in how marginalia comes to be more important than the main text, with Glas being the key book.
Besides the theoretical reasons, there are also practical reasons for writing in margins. It's a good way to keep yourself focused on a knotty text. (Similarly, I take notes--a form of marginal comments--in meetings because I'm bored.) Marginalia helps clarify what you think about something. And one of the best pieces of advice I got in graduate school was to write all the time. Eventually something good will come out.