I don't write in books! I'm a librarian (retired). I sometimes turn pages down, but only if I own the book. I once had to take a library patron to task when I found that he was consistently writing in books we got for him on interlibrary loan from a university library. I think I was pretty polite in speaking with him, but he never used our library again. I was glad! It took a long time to erase all his notes before returning the books.