The book will open better and the binding will remain flexible and not crack. When reading aloud, cup the book in your palm instead of doubling back the cover. The cover will stay attached to the book. Use a slip of paper to mark a page. Objects, such as pencils, are too bulky and split the binding.
Paper clips tear the pages and rust.
Folded page corners break off. Mark a book and close it when you stop reading. Don't flop it face down or set an object on top of it. The binding will crack, and the pages can become soiled. It's too easy to accidentally mark the book and very hard to remove the marks.