A true book weight is a sort of mild figure 8 with weights on both ends. You use it to weigh down both sides of a book when it's open. My sap looks a lot like that except it's hard leather and only one side has a weight in it.

Book weight.

Sap, blackjack sap, slapjack.

My sap is only 8.5 inches long, and being flat works well as a book weight.