From The Los Angeles Times article Amazon Mystery: Pricing of Books, Amazon appears to be experimenting with dynamic pricing:
Imagine this: You go to a bookstore, browse, choose a couple of volumes. But you don’t want to carry the books around. So you ask the clerk to hold the tomes until Saturday, when you’ll come back to buy them.
When you return, the bookseller hands you the items but advises you that he’s raised the prices. “I knew you were hot to buy them,” the clerk says, “so I figured I could make a few extra bucks.”
That’s what it feels like online bookseller Amazon.com Inc. has been doing to me.