Which of the following is NOT considered restricted matter when mailing?

Books are not considered restricted matter when mailing because they do not pose any safety or legal issues that would prevent them from being sent through the postal system. Unlike intoxicating liquors, handguns, and cigarettes, which have specific regulations governing their transportation and sale due to health, safety, or legal concerns, books are generally an acceptable form of mail. They can be shipped freely as they are non-hazardous and do not typically require special handling or labeling. This makes books an entirely permissible item to send through USPS without any restrictions.