If you gave all of the books in one donation to one charity, then you would have to consider it as one collection. It is impossible to come up with the exact price of these books purchased over many years. The most that you can do is calculate a good-faith estimate of what they could have cost your parents originally. If they aren't rare books, the original cost will be higher than their used value, and you will be claiming the lower of the cost or the fair market value on your parents' return. But if these are rare or collectible books, you'd most likely have to consult an expert in this field who could provide guidance for both the value and the original prices.