You can write as many marginal notes as you want in your personally owned copy of a book, if that's what you want to do. But you may not, Charlie, write notes in a library copy—not even if you write in pencil. It isn't yours.
I recently read a library copy of Reza Aslan's Zealot, a biographical study of the historical Jesus, which is full of mind-expanding takes on life in Galilee, Jerusalem, Judah, among the Romans and the temple priests. Halfway through the book I came upon a penciled comment by a former reader of this library copy. With so much controversial information to comment on, this reader chose to scold the author for not capitalizing the word "earth" in a particular paragraph. "Earth is our planet and must be capitalized," he wrote, sideways, in the margin. He had made no comment at all on the wondrous historical facts, the innovative interpretations of the story, the author's amazingly detailed and controversial ideas. No. All of that was completely acceptable to this reader. But "earth" without a capital E? Shame on the author and his editor!
Although I still frown on this book defacer's obvious sense of entitlement, I must admit I'm intrigued by his priorities. If he hadn't felt free to comment on the lack of a capital E, I would never have known this strange duck even existed.
Nonetheless, I will repeat the lesson: write your marginalia in your own books if you must, but never write in publicly owned books. Please.