Gee, I have tons of unread books on my Kindle. I guess I come from the school of thought that says "You can never have too many books", and "You can never have too fast a computer", and "You can never have too much memory in your computer" (the same applies to disc space).
I guess I have around 100-150 unread books. Mostly bargain books and freebies -- around 10% are paid for at full price, and I try to shop around -- sometimes the same book can be listed at different prices - especially public domain stuff. I have very few books that I paid 9.99 or more.
I like to read more than 1 book at a time. My wife says I have the attention span of a knat. So I am currently reading a western and 3 mystery/thrillers, plus the current issue of Analog. I fit right in with the "Samples" idea that Amazon has implemented.. I have about as many samples as I have books, and I love to switch back and forth reading them too. I have only finished 1 sample so far - which I then bought to continue reading - "Black Widow", I like the Sanibel Island series. So far, I haven't finished any of the books - but I will, it'll just take some time.