I've underlined sentences in books (sometimes too fervently) and made odd notes in the margins and between the lines. Oddly enough, I never seem to manage to put pen/pencil to page when there is a genuine typing error in the text. I always catch these mistakes - even in second or third editions of books - and never bother correct them in my own edition, let alone contemplate contacting anyone about fixing it for future ones.
I was in the airport a week or so ago and saw a woman reading 'The Hunger Games.' She was about two chapters in when she dog-eared the page to keep her place. My brain screamed, "THE HORROR! Why can't you just use your boarding pass as a bookmark?" I'm always reprimanding my mum for using the jacket flaps of hardback books to mark her place. She's sure that's what they're for. I am adamant it is not and oft thrust bookmarks at her to keep her from her wicked ways.
Then we have the less-than-common ways of altering or damaging books. My copy of 'Harry Potter and the Half-Blood Prince' has teeth marks in it because I thought it would be fun to read the Snape parts to my cat, Severus. He seemed to appreciate it, perhaps a little too much because in the midst of smooching the book, he decided he would also like to sink his teeth into it. He was less than kind with another book. My mum was taught a harsh lesson when she left her copy of 'The Devil's Feather' by Minette Walters on the floor. Severus urinated on it out of spite.
Books are gorgeous when they're new. Often, when I'm in a store, I make sure I don't pick the copy furthest to the front, in order to get one in the best condition. However, if books are to be read, we must accept that pages may be creased and spines will wrinkle. Sticky fingers may find their way onto pristine pages. My original copy of 'Howl's Moving Castle' had to be thrown out when it fell apart. It was a sad moment when I binned those pages.
What is your limit for the wear and tear of books?