Is it permissible to eat from or read a gift of food or books that was given to someone else, whether it was given to me without solicitation or returned to me after they had taken it? And is that considered reneging on a gift?

If the donee lends the gift to the donor, or gifts back part of it to him, or presents it to him, there is no objection to that. This is not the prohibited retraction of a gift, as it is disliked for charity to return to the giver, because charity is for the reward of the hereafter, unlike a gift which is for the benefit of the donee.