Stock and Supplies can specify a cost and book cost when setting up price scales.
Cost refers to the actual cost being paid for the stock or supply.
Book Cost refers to the price given by a vendor ('cost by book').
In many cases, special pricing has already been worked out with a supplier, so the Cost can actually be lower than the Cost by Book. If you compare the cost to the cost by book on an item, you can tell how much of a discount is being applied to the item based on the cost difference.