What should I do about a book I borrowed from a Christian friend, given that our relationship has been severed and I cannot return it to him? Is this considered a debt tied to my neck?

Trusts must be returned to their owners, even if the owner is a non-Muslim. The Prophet, peace and blessings be upon him, borrowed coats of armor from Safwan ibn Umayyah, who was a disbeliever. Therefore, the questioner must preserve the borrowed book and search for its owner by all means to return it to him. If he does not find him after exerting effort, then he has absolved himself before God, and the book remains a trust with him. If he despairs of finding him, he should place it in a public library or give it to a poor person in need, because its ruling then becomes the ruling of lost money that is spent on the interests of Muslims. The questioner is advised not to befriend non-Muslims, acting upon the Prophet's, peace and blessings be upon him, advice: "A person is on the religion of his friend, so let one of you look at whom he befriends," and "Do not befriend anyone but a believer, and let no one eat your food but a pious person."