Is it a sin or an act of shirk to keep books containing shirk for the sole purpose of preserving their physical condition, and without assenting to their content?

Books of shirk and similar prohibited books must be destroyed. It is not permissible to sell them, nor is it permissible to keep them, because there is no permissible benefit in them. However, if you do not destroy them, it is not considered disbelief (kufr) or polytheism (shirk), but keeping them necessitates sin.