Most people don't think of metal when they think of books. Still a lot of metal parts are used in some types of bindings. Very often clasps are used to make the book shut tight (especially for heavy, thick books). For very valuable books metal was used to fit the gems which were mounted on the binding. Later in history metal was mainly used in a functional way: by means of protecting corners and edges of leather (tooled) bindings using metal strips, or by cylindrical cones to protect the cover leather.
Mostly these parts are executed in brass but sometimes in silver (see example) too.
The example above shows two stages in restoration: a clasp heavily corroded and the same clasp after using ultrasonic cleaning; the fine carvings are revealed again.........