Sadly, some things you can download and install on a device, but they still are set up to 'require' internet access. Games are a big one with that, but many apps are like this. A few years ago I downloaded an app through Amazon (you couldn't get it anywhere else, still can't) and it would not run on the device unless I had the Amazon app on it and WiFi or Data turned on so that app (the Amazon one) could connect with the device. That was the only way I was able to use that app I downloaded, installed and actually paid for. That was the last time I got an app there or had that app on my device. 😉 So, sadly, it is very common. Your only options is to have internet access. WiFi or data, or to change where you get your books from.