When you're finished with an exceptionally good book, for instance, it has sat gathering dust on your shelf for a year, write:
PASS THIS BOOK ON
on the book and write your email address in the book. Then give it to a friend. Tell her to pass it on when she's done.
This is the idea behind Book Crossing.com.
(If american lawmakers (and the lobbiests that buy them) have their way, the Right To Read (www.gnu.org 
) may become illegal under some future version of the Digital Millennium Copyright Act.) -- Ha Ha Only Serious
If you give the book away, you can't read it again for free.
A problem with this is that you miss out on the benefit of re-reading a book over time ("You never cross the same river twice - because you have changed, and the river has changed." (River Of Time))
Maybe buy two copies of a book that impacts you so significantly that you can't decide whether to Pass The Book On or to keep it.
(Excellent idea, I've given away old copies of In Cold Blood and Rule of the Bone, only to buy them again when I wanted to reread them. -- Sean Oleary)
Well, I wrote this topic because I have a bunch of books on my shelf that I have not referenced in years (a few remain unread... never buy more than one book at a time!). I am now inclined to take the writer's advice below and recycle most of them.
Don't pass on mediocre books: recycle them.
Only pass on the good books. If you thought reading the book was a waste of your time, recycle it.
Shouldn't you try to sell it to someone else, first? Just because the book was useless to you doesn't make it useless to everyone.
See original on c2.com