My book stockpile! I get twitchy if I don't have plenty of options to choose from for my next read. I get about 85% of my books from Paperbackswap, 10% from garage sales, then I maybe buy 5% new. I have about 40 different titles on my wishlist at Paperback and usually about 10 credits on hand and this works well to save money since I do so much reading.
Now to decide what I should start tonight!