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.