What is the ruling of Islamic law concerning money taken from a person in exchange for religious books with the intention of returning it, but contact with that person was lost? Is it permissible for the one who took it to keep it, or must it be given away as charity?

There is no blame upon the first person for taking this money. It is better for him to give it as charity, or to buy other books with it and donate them to those who would benefit from them, thereby fulfilling his good intention and preserving the completeness of his reward. This is based on the Prophet's (peace be upon him) statement to Umar ibn al-Khattab when he wanted to buy back a horse he had given in charity: "Do not return to your charity."