Thanks for the feedback. We are well aware that we need to do more work on the UI to make this easy for customers.
The book is downloaded to the browser cache. What this means is that if you come back to the site without internet connection you can read that book or any other book you have made available offline.
It is not however a file or PDF that you can use on another device.
So if for example you wanted to load up the book to read on your iPad for a flight (one day when we allowed to fly again) you would go to our site pick the books you want to read and make them available offline. Then when in the air you can still go to our website as it works offline and read these books within the reader on our site
Hope this makes sense. Again thanks for the feedback we are working on this to make it better for end users.