An example of this (letters) https://nypost.com/2021/11/05/wisconsin-inmates-to-get-photo...
> Wisconsin prison inmates will begin receiving photocopies of their mail next month as officials attempt to stem the flow of paper laced with drugs into correctional facilities.
> The Wisconsin Department of Corrections (DOC) said paper and envelopes can be sprayed or soaked in synthetic cannabinoids such as K2 and then sent into prisons, where they are torn into strips and smoked and sold to inmates.
> The Fox Lake prison switched to photocopying prison mail in April after nine “potential overdoses” and two incidents of violent behavior connected to drug use. In the months since, the prison has seen numbers in both categories drop before reaching zero in August, officials said.
That's for a single sheet of paper - consider the difficulty of verifying that a couple hundred pages of a novel haven't been tampered with.
One of the easier ways to do this is to have the book be new and come directly from a distributor.