When you purchase an e-book it is downloaded to your device. It
is yours to keep. If you delete the book from your device it is
gone, unless you have it stored on your computer. If you download
an e-book from a library, it is yours to use for a specific time
period. For example 1 month. At the end of that month, the
downloaded book will be removed automatically from your device.
This is because the book is not purchased, it is borrowed.