See Petition forum for Change.org link to sign online petition to stop Washington State banning free books to thier inmates.
Just to clarify, Washington DOC does not prohibit sending books and gifts to prisoners and never has. The restriction is to prevent contraband and only applies to non-secure packages. To send books to your inmate, use an approved vendor to send a secure package. Most states have this rule as well. Check out JPay, western union and CorrLinks for better options.