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: At the ends obviously but if placed inside the cover you don’t have to see the ends and you can enjoy the seeing the whole front (or back) of your books.