A "prison library" refers to a library that is found within a prison. It is a place where inmates can borrow books and access educational materials while serving their time. Full definition
So the bureau set up a task force to deal with all the books in prison libraries, or at least all the books that are «religious» in nature. (faith.org.uk)
After a few more years, he receives enough money and donations to build one of the biggest prison libraries in the nation. (inc.com)
You often hear of prisons with a really good prison library - but without enough staff to actually take inmates to it. (politics.co.uk)