There are two sorts of people in the world: Those who write in and mark up books; and those who view those of us who do write in books as sacrilegious pigs.Okay, okay—maybe there are a few other sorts of people. (I've never been a fan of that whole "There are two kinds of people" routine, except where it is inarguable: women/men; living/dead; rational people/fans of Glenn Beck.)Myself, I've gone from treating every book as a sancrosanct object (as a boy) to routinely scribbling in books (as an adult). Some I so love that I want to puzzle out how they work, and I buy multiple copies and mark them up (Moore, Munro, Cheever, Konigsburg, others). Some books I find so maddening that I have to immediately vent my hooting disdain (among them recent award-winners and bestsellers—don't ask). Years later I'll be flipping through an old copy of something and find an expletive in a margin and think, "Really? Was it that bad?"But my marginalia is as nothing compared to the marginalia of the greats.There is a wonderful little piece by Ian Frazier in this week's New Yorker about the marginalia in books owned by famous writers, among them Nabokov, Coleridge, and Twain—who probably wrote the most entertaining marginalia: "At the end of an unusually exasperating chapter, [he wrote,] 'A cat could do better literature than this.'"So writing in books: Bad? Good? A necessary evil? A perversion that must be stamped out? Do you write in your books?