If you are still reading paper books, i.e. you haven't bought your Kindle yet, how about swapping out the ones you have read for some ones you want to read? What if you could swap out your books for the cost of postage?
There is a free service called Paperback Book Swap which coordinates the exchange; members pay for postage (the cost is about $2.25 to ship one regular sized/weighted paperback) to ship the books to other members, but remember that when you receive your books, you pay nothing, so this is truly an even swap. It works a bit like a book version of Ebay, but without the bidding process; swappers simply ask for the books they want and those who have them ship them out, so everyone is both a buyer and seller at the same time. The only restrictions are that you must be over 18 years old, not incarcerated, and have a valid email address and USPS shipping address in the Continental US.
There is a handy postage label link on the site so you can simply drop your book off at a mailbox if it weighs less than 13 ounces. You are not limited to paperbacks; you can swap audio books, DVDs, and video games using the same system. They will even give you 2 free credits (each credit is equal to one book) when you sign up.
Swapping your books and CDs for free won't last long (will paper books and CDs still be here in 10 years?), so take advantage of the opportunity to enjoy our old fashioned ways while we make the transition into electronic medias.