Books to Prisoners is a Seattle-based group that provides books for prisoners. You can donate books--they're always looking for dictionaries, books on auto repair, on legal self-help, on Spanish and American Sign Language instruction, and African-American fiction and non-fiction, as well as westerns and horror--but because their warehouses are full, they prefer money. One of the most requested books is Sun Tzu's The Art of War, and they also have an ongoing need for true crime books.
This is an organization where a modest gift can do a LOT. Just twenty-five dollars sends a package of books to seven prisoners. One hundred dollars covers the postage for an entire day's worth of requests. (This year, Books to Prisoners received 13,000 requests for books.) Check this organization out and consider donating.