When Ron tore down the most recent Play Store APK, he discovered references to pre-orders hiding within. It wasn't clear at the time just what this option would apply to, but now we know! It's books! According to a new help page, if an item on Play Books is not yet available, you may find a pre-order option that you can use to place a reservation. Neat!
The page also states that when you place a pre-order, your credit card will not be charged until the book is available. This may be nice for some people who would prefer to pay later rather than sooner.