I understand your upset. Who wouldn't want their child to enjoy books?

Child abuse is harm to, or neglect of, a child by another person, and you might argue that actively denying the child books is either criminal neglect or emotionally abusive. Depends on the situation.

Did you loan him your books? If so, you can call the mother and ask that the books be returned. Maybe you'll get some information from her on what happened to them.

I didn't get a "good feeling" about the mother taking the books and now he can't find them. Sounds fishy.

Poor kid.
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May The Hair On Your Toes Never Fall Out

--Traditional Hobbit Blessing