The books all had clasps (like the one on the right) originally, to help keep the pages flat.
These books are very old (medieval in some cases, very precious, and definitely not for altering!) but the urge to open one was overwhelming. Scholars can apparently access them, but not the likes of me!
This is a modern stained glass window in a side chapel. Unfortunately, my image doesn't quite capture the vivid colours as the light streamed through the glass. They were very beautiful.