I preorder hardcovers, not ebooks, from Amazon and the language Issybird quoted has been around for as long as I can remember. Some books do not get a price guarantee, although it has been very few. I have now started using an application called
Honey Droplist to track prices. It lets me set the length of time (up to 120 days) I want to track an item's price and how much of a discount has to happen before it sends me an alert. I have been using it for about 4 months as a Chrome add-in/extension and it works very well. I will often have it track a price at both Amazon and Barnes & Noble. Honey also will give you a product's price history. I'd suggest checking it out.