Everything you have ever ordered for your Kindle is stored in Amazon's Cloud and on your device when you order a book. Always read your books with the Wi-Fi off to save power. When you're finished with a book and want to remove it from your device, turn on your Wi-Fi, hold down your finger on the book, choose "delete from device" and it will be removed from your device, but it is still in the Cloud. If you want it back, turn on your Wi-Fi setting, find your book, tap on "Cloud" to see your books in the Cloud, then place your finger on your book. You should see a prompt to download back to your Kindle. Alternatively, you can go on Amazon under "Manage Your Kindle" and do the same. This is a very helpful feature because, if your Kindle is damaged you can return it to Kindle, get a new one and Amazon will download all of your library into your new Kindle.