If you delete from the Kindle app, it will remain in your archive in case you want to redownload. If you want to permanently delete a book from your account, go to Amazon's website (you can do this from a browser like Safari on iPad) and use the Manage Your Kindle page and click on the drop-down next to the book title, then choose Delete from library. If you delete the second way, it's as if you never bought the book. But you'll have to pay again if you want it, unless the book is free.