And yes, the books you no longer want can have a happy ending. We found three worthy organizations dedicated to finding new owners for your old titles.
Books For Soldiers
Soldiers (and sailors, and Marines, etc.) make requests in an online forum, and you send literary care packages directly to them.
Write a Prisoner
This regularly updated site posts requests from prison libraries. Needs range from law dictionaries to Spanish-language novels, so be sure to consult individual listings.
International Book Project
For 40 years, this non-profit humanitarian group has been sending books to orphanages, schools, and churches worldwide.