Sometimes a library has to get rid of a book. Maybe it is worn out. Maybe there is a newer edition. Maybe it is a 1988 manual on word processing and no one would look at it on a bet.
What happens to those books afterwards? It's nice to think they go on to A Better Place. And sometimes they do. Sometimes people find a way to reuse them. See this wonderful article in Colleges and Research Libraries News on the RE:BOOK Contest. (Okay, this is the 2012 poster. I couldn't find a copy of the 2013 poster that would load. But the new one is in the article.)