Bill Vallicella has a short post beginning with the question
"Have you noticed that the same people who are morally obtuse enough to underline and annotate library books tend to be the same people who are too intellectually obtuse to make good comments?"
A reader of his also has a follow-up in which he notes that
"There's another problem with these people: they annotate the book and then they return it, which not only annoys others, but also renders their droppings unavailable to serve their own future readings."
The whole exchange reminded me of a passage from C. S. Lewis:
"He [Milton] is writing epic poetry which is a species of narrative poetry, and neither the species nor the genus is very well understood at present. The misunderstanding of the genus (narrative poetry) I have learned from looking into used copies of our great narrative poems. In them you find often enough a number of not very remarkable lines underscored with pencil in the first two pages, and all the rest of the book virgin. It is easy to see what has happened. The unfortunate reader has set out expecting 'good lines' -- little ebullient patches of delight -- such as he is accustomed to find in lyrics, and has thought he was finding them in things that took his fancy for accidental reasons during the first five minutes; after that, finding that the poem cannot really be read in this way, he has given it up." (Lewis, A Preface to Paradise Lost [1942; London: Oxford, 1965], 1-2)
I myself have in many used bookstores double-checked Lewis's observation, and I don't think I've ever found a copy of a narrative poem marked up past the first chapter.