Must a book of sorcery be returned to the deceased's wife, or should it be destroyed, knowing that it might be used to harm others, and would the deceased incur sin if the book were returned?

You are not permitted to return books of magic to their owners. Rather, it is obligatory to destroy or burn them due to the disbelief and misguidance they contain, and there is no guarantee for them. A number of scholars have issued fatwas to this effect, such as Ibn al-Qayyim, al-Nawawi, and Sheikh Ibn Baz. Books of disbelief and magic have no value, and their destruction does not necessitate compensation.