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. On the other hand, it does mean that you will be setting up a surprise charge on your account later, so make sure your personal budget takes that into account.
Unfortunately, the pre-orders aren't available for devices, like we were hoping. However, given the clusterf-*ahem*... mess over on the Play Store right now with the Nexus 4 (again), maybe it's for the best.
Update: Here is what book pre-orders look like: