This may be of interest to those freethinkers who are short on time, but want to volunteer for the secular movement on a regular (monthly) basis. You can anonymously discuss atheism and share favorite resources with a prison offender who would otherwise have a very slim chance of being exposed.
The webpage also has a link for how to help donate freethought books to prisons.
I worked in Colorado correctional environments for 4 years and know how badly offenders need a secular alternative to all of the faith-based outreach they receive. I also work with parolees and former offenders who continue to face enormous obstacles on the outside. They will absolutely need the responsible, ethical and empowering guidance and resources you provide when they are released.
HOWEVER, CFI gives sound warnings about not getting too personal and maintaining boundaries. Incarcerated individuals will often take advantage of a helper's good intentions. What would you do in a cage? Whether it's innocent or malicious, you don't know. Err on the side of safety.