you don't transfer books from one device to another, you register the new device to the same account as the old device, turn on wireless and sync and the books will be available to download in the Books tab under All and all you need to do is tap on them and they will download.
I have a new Kindle fire 8 with Alexa and want to transfer my books from my old E - reader to this Can anyone assist in how to do this
Adding to CBR's reply. If you no longer plan to use the old device, de-register it from your account to avoid any confusion. However, if you leave both registered, update your Settings to make the new Fire Tablet (not Kindle) your default device. On amazon.co.uk: