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Amanda, check your transaction archive on the Your Account tab to see what transactions you've had so far. Also you can check your credit registry (also on the Your Account tab) to see exactly how your credits have been spent.
If the book you ordered was an audio book, it costs two credits, not just one. Also, if you've requested a book but the order hasn't been accepted yet, it will "tie up" your credit until you either cancel the request or it goes through.
You can buddy credits to others, but I'd imagine that you'd remember that... Did you sign up for a swap in the games forum and forget about the credits for them? (I've been there and done that, LOL!)
It could also be that there was just a delay in processing and they'll show up in a few minutes. It happens sometimes.
Hope this helps!