What should I do with a book I borrowed from a Christian friend whose address I no longer know after I severed ties with him due to his aggression towards the religion, in order to return it to him?

The book must be kept for its owner, or for whoever will deliver it to him. This is part of fulfilling the trust, as Allah Almighty says: "Indeed, Allah commands you to render trusts to whom they are due," and the Prophet (peace be upon him) said: "What the hand has taken, it is responsible for until it returns it." If you despair of reaching him, it is permissible to dispose of it, and you must return its like or its value if its owner is found. One should deal with such people with wisdom, good admonition, and gentleness, for ignorance of the religion can be a reason for hostility towards it. Be patient with their harshness, for guiding one man is better than red camels.