Two ways of using simple metal bookends:
#1: in the center of a shelf to hold books in place when a book is taken out. This is so helpful for those with long bookshelves. I often refer to my Housekeeping Handbook and since it's quite large, the books to the left of it used to fall over when I take it out, not anymore.
#2: I didn't like the bookend covering the cover of my book on Steve McQueen (sorry, that's on another shelf), so I placed the bookend inside the cover. For use with a paperback, I would recommend using a large binder clip to hold the cover on.