As you would expect, the Play Store is the primary source of apps for Googlebooks, and there will be some limits on sideloading. Google has confirmed to Ars tha

As you would expect, the Play Store is the primary source of apps for Googlebooks, and there will be some limits on sideloading. Google has confirmed to Ars that Googlebooks will enforce developer verification requirements for apps similar to Android phones. OK, I'm a little behind the latest in terms of what's going on in Android land. Could somebody who's more up to date say what the current state of side-loading is? I mean specifically when you have physical access, a USB cable connected, and using ADB or whatever.