Maybe a strange location for putting book clasps as a sub-chapter of Leather Belt plating&decorations - truth is many worn items are most likely book clasps. Some are damaged and could have been items such as Other Clothing items....or even parts of Buckles. Nevertheless some are definitely book clasps! Ancient books held leather and or wooden covers. These books held mostly heavy weight and many pages, resulting in a thick book. These books were held together and closed by metal clasps. These clasps consisted of multiple pieces and different  types of sets existed. Many parts can still be found. 

Above two parts of bookclasps 'halfronde muiter' PAN 23-01-11-02-05, dated 1400-1700. Normally these would have been attached by holes or rivets, but these would have been glued (resin) between the cover leather/ wooden plates.

The picture to the left above shows the mechanism as was used on the finding below. Different models used different types of mechanisms - almost all you can imagine!