There is a difference between deleting and returning an ebook. Deleting a book removes it from your device, but doesn't return it to Cantook Station. If you delete a book, it will still show up as an active loan in your account and you can still redownload it until the end of the lending period. If you have deleted a book and would like to return it, you need to download it again from your account, and then return it from your device or e-reading app.