I started releasing books into the wild a year or so ago - then discovered a place that was thirsting for books : the Veterans Hospital, especially "guy" books - action, mystery, westerns, etc.
My local library has a rack of used paperbacks that may be purchased for 25 cents each and I raid that when I have a few bucks extra. The Red Cross and Salvation Army also can find new homes for books. If you know someone serving in Armed Foces of your country, a box of books is always welcome.