Are you a spine breaker? Or a dog-earer? Do you expect to keep your books in pristine condition even after you have read them? Does watching other readers bend the cover all the way round make you flinch or squeal in pain?
That's the best part about reading library books - they already come equipped with the torn pages, chocolate smudges, bent corners, and frayed spines. If the book is new, then I have a lot of work to do! The only thing I won't do to a book is write anything in it - notes in the margins, underlining words, correcting grammatical errors, and even *cringe* highlighting stuff. It's the author's story, not mine! What they said is what they said and you shouldn't add to it or take away from it. That kind of annoys me while I'm reading and takes away my concentration from the book. Especially when the passages that they are underlining are mild parts that there is no reason to be underlining.
I read a book once where the person wrote questions to themselves all throughout the book - "What is the author saying? What does this word mean?" Translating words into another language and crossing out grammatical errors. And underlining passages that were so puzzling to me - why in the world did that person underline that when the next passage is so much more interesting? The actual story of the book faded into the background while I was dwelling on the person's inane comments. It was almost like reading two books at once.