Hi Neil! I also volunteer with a prison book program. Prison Book Program in Quincy, MA is one of the oldest in the country, and sends books to all 50 states as well as prisons in Puerto Rico and Guam. They have a list of other programs here as well: https://prisonbookprogram.org/prisonbooknetwork/

Prison book programs are so, so necessary because US prison libraries aren't required to provide anything but legal texts, but books are a lifeline for incarcerated people. It's some of the most fulfilling work I've ever done!