I frequent used bookstores. A lot. I have a set of authors (Charles De Lint, Samantha Hunt, Brian McLaren, Frederick Buechner, R.S. Thomas, etc.) and subjects (theology, faery, goats, Regency England, Romania, etc.) that I'm always on the lookout for, and I just like looking at books.
One of the unique pleasures of acquiring used books is observing the occasional mark of the previous owner. Besides the typical name or birthday wish scribbled in the front of the book, sometimes you get to be privy to the inner recesses of that previous owner's mind as you read through the book and encounter his/her notes scrawled in the margins, usually accompanying the abstract art of lines and stars highlighting certain parts of the book that s/he found especially arresting. It's even more interesting when the notes and highlighted sections make no sense.
I just got a cheap paperback titled "Faery!" Whoever owned this book was a happy highlighter. On page 115, under the story The Seekers of Dreams, by Felix Marti-Ibanez, the words "vice versa" are underlined. On the page opposite, the sentence "I was afraid to quit the security of my prison" is underlined, with a careful circle around "I," with a tiny note to the left "& me?"
In other stories, phrases like these are underlined: "the faint glow of magic, like infant dragon fire," "one who had lived as long as the rivers and mountains, and possessed the knowledge of death for every day of it," "he loved her in desperation and silence," and "Chinese unicorn."
Our happy highlighter was also only too happy to add commas and dashes and semicolons, and even insert words here and there, as though the authors of the stories had been careless in their writing. In the first sentence of the story "Touk's house," by Robin McKinley: "There was a witch who had a garden," our self-made editor inserted "once" between 'was' and 'a' so that the sentence would read "There was once a witch who had a garden."
While this person did not indulge in too much marginalia, it was still amusing to see such liberal use of underlining, particularly in a collection of fantasy stories published in a cheap paperback.