"The Care of Books
5. Don't use a thick book-marker, whether it is your morning's post or an elegant leather affair. Miss Dorothy L. Sayers has said: 'Please burn all your book-markers - even the pretty one Aunt Mabel sent you last Christmas (or at least put that one away and only bring it out when she comes to call). You cannot possibly be so bird-witted as to be unable to discover which page you got to by looking at it." If you are reading The Wealth of Nations, and do not feel you can follow this counsel of perfection, use a piece of ribbon or a thin piece of paper. Anything thicker will make unsightly bulges where the marker has rested for any length of time."
[Dorothy Sayers' quote from Begin Here]
Books of Your Own Elizabeth Edmonston, National Book League