How do I cancel an iBook? What happens when it is less expensive on Amazon?

If you mean cancel a pre-order then you can do so on your computer's iTunes : http://support.apple.com/kb/HT5714

If you have already bought and downloaded the book then it's too late to cancel, though you could try contacting iTunes Support and see what they say : http://www.apple.com/support/itunes/contact/ . The book publisher sets the price for the ibookstore and for Amazon.