Ans- This feature was added to prevent the misuse of the eBook. Once you log out, the book will disappear. You will have to re- enter the codes and download the book again.
However, we are working on the app to get an update soon in which if there is same device login, your eBook will be still there, but if the device has been changed you’ll still have to re-enter the book codes. The new version will be available soon on the Play Store.