> While it is becoming widely acknowledged that long-term solitary confinement is essentially a form of torture, in theory it's not about causing suffering, but rather about protecting inmate safety. Denying books wouldn't really make sense in that context.

That seems to go against what Wikipedia says? It says it's primarily about punishment rather than safety:

> [Solitary confinement] is mostly employed as a form of punishment beyond incarceration for a prisoner, usually for violations of prison regulations. However, it is also used as an additional measure of protection for vulnerable inmates.

I didn't have the resources to purchase outside books during my 2 year incarceration but it was remarkable how outdated the "jail library" books were. I read quite a bit of cosmology and physics, mostly 30 or so years old. A personal highlight was finding a Ray Kurzweil book.