Intellectual Freedom in Prison

Do prisoners have the same First Amendment rights as everyone else? This question has been on my mind for a while now. As a bookseller, I frequently help people who want to send books or magazines to someone in either the county jail or prison. Recently, a customer came back after purchasing and sending a few books that were religious or spiritual in nature. The books were refused by the prison and the customer needed detailed descriptions of the books in order to file a formal complaint with the prison.
There are two sides to the question of prisoners’ rights. On one side, some would argue that someone who has committed a crime and been convicted has surrendered their rights. On the other hand, a prisoner is still a citizen of the United States and should be protected. The reality lies somewhere in between. The Anti-Defamation League states that “The United States Supreme Court has said that ‘prison walls do not form a barrier separating prison inmates from the protections of the Constitution.’ (http://www.adl.org/civil_rights/prison_ex.asp#q1).” But these protections are limited for security or disciplinary reasons. So, an inmate has a right to read what they want, as long as the prison officials say it’s all right.
But how do prison officials judge content? What kind of book would pose a security risk in prison? ADL talks about prison extremism—literature from groups that promote racial hatred or a challenge to authority. This makes sense; there are definitely racial issues in prisons, and prison guards also need to be safe. But in the case of my customer, the books were not in any way inciting violence. So what criteria do prisons use to determine whether books are ‘appropriate’?
When I began researching this question, I felt very strongly that prisoners should have the same first amendment rights as I do—being in prison doesn’t strip you of your constitutional rights. I think there are very few books that would pose a security risk, and those books would probably not be found on the shelves of my bookstore. I guess I’m left with more questions than when I started. I’m curious about prison libraries: Do they have the same sort of collection development policies as other libraries? Can they support the Library Bill of Rights or the concept of intellectual freedom? What does intellectual freedom mean for someone who doesn’t have physical freedom?