Grab a few books you've read and have lying around. Next time you go to the launderette, doctor's office, or anywhere that people sit around and wait, just leave the books there. There are, of course, organized ways of doing this. BookCrossing.com is one example. Then there's Books for Soldiers. But me, I'm all about doing things the easy way. I just leave mine at the laundromat.
A suggestion: if you can bear to part with them, try leaving good books ; )
I've heard stories where books have changed peoples' lives. One woman I know - a single, working class mom- actually was inspired to go to college at age 45 as a result of finding a book in the toll booth where she worked, and wound up being valedictorian of her class. She had more job offers than she knew what to do with, and all kinds of self esteem etc. that she'd never had before. (I didn't like her personally, thought she was annoying, but good for her anyway).
Ain't it funny how little things like that can change a person's life?