If you long-tap a book's title, you will see that Remove from Device is the last choice on the menu that appears. If you purchased the book from Amazon, it will be available in the Cloud, which is your archive of Kindle books at Amazon. If you remove the book from the device, it is still available to you to download via the Cloud.
However, if you have purchased a book from another source, it will not be backed up in the Cloud, unless you have sent it to your device. In this case, when you remove the book, it is permanently deleted unless you have backed it up on your computer.