Yes, books have furniture. Well, some of the larger, older ones do. Though if you kept a diary as a teenager, chances are it had a little clasp with a keyhole, so that you could lock your diary and prevent the prying eyes of your brother or sister from reading all about your deepest feelings about Brandon from Beverly Hills 90210. We can call that furniture.
Furniture was partly decorative and partly functional. Corner plates protected the corners of the book from accidental damage. Clasps allowed the books to be locked. A boss was a rounded metal piece inset into the cover. Usually a book had several bosses, meaning that when it was laid flat the book would rest on these elements rather than on the leather covering, preventing scuffing. Also, they look rad.
Various metals were used to make furniture, often brass. Sometimes you might see green corrosion around these elements, where the metal has reacted with the leather or (more likely) a leather dressing. Green corrosion is generally indicative of copper, which is used to make brass and bronze.