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And don't order used books. Only new books. Used will get rejected.

Huh, that’s surprising. I’m curious, do you know reason for that?

My understanding is "anything not new might be modified". It's not specific to books.

"Modified" as in, contains contraband that gets smuggled in via the book.

Prisons have lots of (seemingly to me arbitrary) rules. Only new books from the printer has been my experience.