What is the ruling on buying original books and then disposing of them after photocopying them, and what is the ruling if one is unable to contact the copyright holders?

If you are unable to reach the rights holders to deliver their rights to them, you may give charity on their behalf. As for buying copies to destroy them, there is no benefit in it, and it is considered a waste of money.

It is worth noting that photocopying books for study and personal benefit, without gaining profit from them, is a lesser matter, and some scholars permit it and do not obligate anything for it. According to this view, you are not required to give any charity, but you should stop doing so in the future.