It's hard to inspect books. Books have been used to pass codes since their early creation. If an inmate is still engaged in criminal operations, passing books

It's hard to inspect books. Books have been used to pass codes since their early creation. If an inmate is still engaged in criminal operations, passing books back and forth makes it easy to still operate. Also there is some risk of the book containing contraband. You can x-ray them but that doesn't catch everything, especially not narcotics.